CEO: Dipanshu Sharma

Estimated gross revenues: $65 million, net revenue would be less than that.

Employees: 102

Total venture funding: $24 million

Comment: xAd tracks in-store visits from people who recently saw mobile ads, allowing clients to do hyper-local mobile targeting and location-based "retargeting" (hitting you with an ad just bceause you were recently near a relevant store).

We believe xAd is profitable. Its clients include Macy's, Home Depot (direct) and YP. xAd also gets revenue from platform partners using xAd for their long tail. More importantly, xAd is the exclusive third-party provider of search and display mobile ads for Waze, Google's new mobile traffic mapping app.

CEO: Simon Khalaf

Estimated revenues: $100 million

Employees: 140

Total venture funding: $50.5 million

Comment: "I consider an IPO an entrance," CEO Simon Khalaf told us. "We don't have a choice, our volume is too high and our scale is too big for anyone to absorb us."

CEO: Ragy Thomas

Estimated revenues: unknown.

Employees: 300

Total venture funding: $28 million

Comments: Sprinklr targets large companies that need massive social, CRM and enterprise communications management needs. (In plain English, imagine a company that has a different Twitter account for every single product it market in every single country. Now multiply that across all the other platforms companies need to use to communicate — LinkedIn, Facebook, etc. Global companies can easily end up with hundreds or thousands of social media accounts. Sprinklr's products can sit on top all of, that making them much easier to manage.)

So far, Sprinklr has flown mostly under the radar.

The company acquired The Dachis Group earlier this year for an undisclosed sum. TBG Digital — one of Facebook's larger ad partners — is now Sprinklr's de facto Facebook ad-buying arm.

CEO: Jan Rezab

Estimated revenues: $30 million

Employees: 300

Total venture funding: $34 million

Comment: Socialbakers is one of Facebook's larger international ads partners. It "would be profitable immediately if we were not investing in scale, growth and international," Rezab told us. The company is growing its offices in New York and San Francisco (it's based in Prague). SB also has 1,500 clients, including Nestle, Nissan, HP and eBay.

CEO: Yaron Galai

Estimated revenues: $100 million in 2013

Employees: 318

Total venture funding: $99 million

Comment: This content-recommendation company is expected to file an IPO imminently. It previously hired a CFO with IPO experience. The Israeli press has previously valued Outbrain at $1 billion, although more recently it said the valuation was a more modest $350 million.

CEO: Mike Baker

Estimated revenues: $100 million-plus

Employees: 235

Total venture funding: $60 million

Comment: DataXu CEO Mike Baker tells Business Insider that yes, his online ad-buying company will go public. An IPO is a matter of time — and timing — he says.

DataXu is one of the larger independent online ad buyers, or "demand-side platforms" as they're called in the business. It had revenues of $87 million in 2012 and was on course to do "well over" $100 million in 2013, Baker says. The five-year-old company is one of Facebook's ad exchange partners. It has taken more than $60 million in venture capital funding, but Baker declines to disclose what valuation DataXu's backers have put on the company.

CEO: Tim Cadogan

Estimated revenues: $135 million +

Employees: 313

Total venture funding: $75.5 million

Comment: Positions itself as a software-as-a-service platform company that offers ad serving, real-time bidding buying, and publishers. The company hired a new CFO in February, which is frequently a pre-IPO move.

Company says it is profitable and Cadogan tells us he declines to rule out an IPO. The company has taken so much venture funding, however, that a non-IPO exit would require a massive valuation attached to it considering the company's current revenues.

CEO: Mike Cassidy

Estimated revenues: $200 million

Employees: 343

Total venture funding: $40 million

Comment: Undertone is an unusual company in adtech in that it is not concerned so much with automating the process. Rather, it has a creative, media and tech platform that allows advertisers to create one video campaign that will work across all screens. The company has added nearly 60 staffers since we last placed them on this list.

CEO Cassidy says Undertone has been profitable since it was bootstrapped at its founding in 2002. He doesn't rule out an IPO, but says it's "better to be a private company today." Our sources tell us that an IPO is one option on the table.

CEO: Rajeev Goel

Estimated revenues: unknown

Employees: 411

Total venture funding: $76 million

Comment: PubMatic offers publishers a real-time bidding media sales platform. It reportedly hired banks to advise it on an IPO valuing the company at $1 billion in February. It hired a CFO with IPO experience in late 2011.

CEO: Yoav Izhar-Prato

Estimated revenues: Unknown

Employees: 393

Total venture funding: $50 million

Comment: Kenshoo took another $20 million investment funding in April.

CEO has claimed that Kenshoo handles $25 billion in spending through its search, local marketing, and social media platforms. Kenshoo is also one of Facebook's FBX ad exchange partners, and probably one of Facebook's single biggest ad spending clients generally.

Kenshoo is also the single biggest buyer of Google shopping (PLA) ads, according to Jefferies Research. Kenshoo runs ads for 40% of the top 50 e-retailers.

CEO: Aaron Bell

Revenues: $150 million

Employees: 467

Total venture funding: $89 million

Comment: The company recently took a $70 million round of investment funding. Levi's, Salesforce, and Skullcandy have been among its 15,000 clients. Techcrunch wrote recently:

AdRoll and its investors are not disclosing the company’s valuation in the wake of the new funding, but Charles Moldow, a general partner at Foundation who has helped advise the company for years now, says that it puts AdRoll on par with other major ad tech companies on the market today such as Criteo. As of right now, publicly traded Criteo has a market cap of $2.17 billion.

CEO: Scott Ferber

Estimated revenues: ~$200 million in 2013

Employees: 325

Total venture funding: $130 million

Comment: Videology has taken such a massive amount of funding that it's hard to see how the company's investors will get a premium on their equity without an IPO. Ferber has recently denied he intends to go public. (Of course, the mere fact that he's being asked that question is all part of the dance ...) But he told Business Insider recently that the company would be "prepared" to IPO in 2015 if need be, even though a decision on whether to do that has not yet been made.

Videology also made a recent measurement deal with Nielsen.

CEO: Joe Apprendi

Revenue: ~$200 million (net revenue is a fraction of that)

Employees: 400

Total venture funding: $56 million

Comment: The company has taken a lot less funding than its peers, making its exit strategy for the remaining shareholders likely a lot more lucrative, ultimately. We believe the company is profitable. "We're one of the few companies in Silicon Alley that actually pays taxes!" Apprendi tells us.

Collective will soon move its HQ into the old New York Times building.

CEO: Bill Demas

Revenues: $500 million, estimated.

Employees: 417

Total venture funding: $138 million

Comment: The company recently raised an $80 million investment round. Demas was coy about going public when he talked to Business Insider: "I wouldn't rule anything out. We're always open to any potential option."

He also told Techcrunch: “I don’t have a crystal ball. Our focus is on building a great company. Obviously, a fund raise like this allows us to have more flexibility about when we go public.”

The company has reportedly been valued at as much as $750 million.

CEO: Brian O'Kelley

Estimated revenues: Greater than $50 million, and maybe as much as $200 million.

Employees: 550

Total venture funding: $141 million

Comment: AppNexus handles at least $700 million in adspend a year, and probably greater than that at this point. Given the size of its employee headcount, the publicly cited revenue number of $50 million-plus seems very low. The company is such a big handler of online ads that it may be too big to be acquired, which is why it is the subject of constant IPO rumors.

It's probably twice the size of Rubicon Project, which went public with annual revenues of around $112 million based on its Q4 numbers.

CEO: Naveen Tewari

Estimated revenues: $372 million

Employees: 900

Total venture funding: $216 million

Comment: InMobi is now one of the largest mobile ad businesses that has not gone public. Our back-of-the-envelope math suggests InMobi's annual revenues could be ~$372 million annually. IDC puts InMobi's U.S. revenue alone at ~$40 million. Note that the company is not likely to be profitable, based on its employee base and this revenue estimate.

Also, investors may have soured on mobile ad network IPOs following the Velti disaster.

CEO: Ben Silbermann

Estimated revenues: unknown.

Employees: 333

Total venture funding: $338 million

Comment: Pinterest is about to launch its Ads API, the portal that will allow adtech companies to plug in to the Pinterest platform and begin serving ads at Pinterest users. Notably, both Facebook and Twitter did the same thing months before their IPOs. Pinterest already provides up to a quarter of all incoming traffic to online retail and e-commerce sites.

We looked at the following factors when we adjust our ranking and publish a new version of the list:

Revenues: This is the single most important factor in our ranking. Companies with robust businesses have revenues they can talk about in dollars (not blind percentage "growth" claims). Companies that are modest about their revenues are usually modest for good reason.

Total employees: Companies tend to hire more people because they're handling more business. Headcount is a good proxy for growth.

Funding: Investors tend to want their money back. So companies that have taken a lot of investment funding are under greater pressure to IPO than those that have not.

Reputation: It's great that some companies like to grow quietly without the distraction of the media spotlight — we're happy to ignore them.

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