Instagram just gave us a hint about how big its ad business has grown

About five months ago, Facebook turned on the money jets for Instagram by opening up the photo-sharing site's ad network to advertisers of all sizes and letting them use Facebook's ad targeting tools for the first time.

And today, Instagram is finally sharing a little hint about how the business is doing: It now has 200,000 advertisers, 75% of them from outside the US.

For comparison, Facebook's flagship social network has 2.5 million advertisers. But the company announced in its Q4 earnings, that 98 of the top 100 Facebook advertisers are also on Instagram.

Facebook still won't say exactly how much money Instagram generates, but the latest advertiser count provides more evidence that the photo-sharing app is developing into a real business.

COO Sheryl Sandberg also said on the Q4 earnings call that although some advertisers will likely choose to focus on one service or the other, many will spend money to advertise on both Instagram and Facebook.

Instagram, which has more than 400 million users, is more appealing to younger audiences than Facebook according to Business Insider's recent survey of teens.

"We've been able to engage the younger millennial customers with Instagram," Erika Shumate, cofounder of a fragrance startup called Pinrose, tells Business Insider. She says that Pinrose's Facebook ad focus is more conversion-centered, while its Instagram ads are more brand-based and intimate - when comparing her spend on both platforms she says 80% goes to the photo app versus 20% to Facebook.

Instagram also been ramping up its video capabilities.

Wall Street is increasingly looking to the photo app to become the new revenue growth engine for Facebook. After Facebook's earnings, Barclay's Paul Vogel estimated that Instagram revenue will surpass $1.3 billion in 2016.

Here are some of the other stats Instagram bragged about in its blog post:

• A November Instagram-issued survey showed that 60% of users say they learn about products and services on Instagram and 75% will take an action - like visiting a site or telling a friend - because of a post.
• Brazilian business PetLove saw a 30% lower cost-per-install for its mobile app install ads on Instagram compared to on other digital channels.
• And US-based company called Framebridge targeted people who had visited their site in last 30 days, and those who had added a frame to their cart, but didn't check out in the last 60 days with their campaign. Through a series of photo ads that included a Shop Now button, Framebridge saw their cost to acquire a new customer decrease by 71%.