$2.8 billion Slack has turned down 8 to 10 acquisition offers

The business communications startup Slack launched less than two years ago, but has already cinched a shiny $2.8 billion valuation and a handful of acquisition offers.

Founder and CEO Stewart Butterfield said on stage Re/Code's Code Conference earlier this week that the company has already denied between eight and ten solicitations.

And that's only the number of real offers - Butterfield has swatted away a number of interested parties in their introduction phase, too.

"No one ever calls you up and says, 'We'd like to acquire you - period,'" he explained. "It starts with, 'Hey, we should catch up!'"

Interviewer Kara Swisher asked what Slack would say to a $50 billion phone call.

"I would have to think about it," Butterfield replied. Faced with Swisher's incredulity (and insistence that if he gets that kind of offer he should take it), he said that he's going to make more money than he needs in any outcome at this point - those sort of stakes are more important to new employees with less equity, which is why he'd consider it - and that believes that the opportunity that Slack has right now is really big.

"We've been growing 5% a week for 70 straight weeks - 98.5% of people who have ever paid for Slack are still paying for it," he said. "We'll never have an opportunity this big again. If we want to see how far we can take it, this would be the time."

Butterfield has personally been through the acquisition process before. He cofounded Flickr, which sold to Yahoo for a reported $35 million in 2005, much less than it could have fetched had the team waited. When Butterfield said on stage that he's learned a lot from the sale, Swisher joked that that was like touching a hot stove and saying it taught you something.

Slack has been in rocket-ship-mode this year, adding $1 million in contracts every 11 days and recently raising a $160 million round at that $2.8 billion valuation. Butterfield wouldn't name-names for the acquisition offers, but any company capable of making a play for such a highly-valued startup has to be pretty big (off the top of our heads we wouldn't be surprised by the likes of Microsoft, Oracle, or Facebook).

Deciding whether to sell or stay independent is a tough decision any founder must make, and, as in all things, hindsight has 20-20 vision.

People thought Snapchat was crazy to turn down Facebook's $3 billion offer in 2013, but now it's raising a fresh round at a $16 billion valuation and CEO Evan Spiegel said on stage at Code Conference that he plans to eventually IPO. On the other end of the spectrum, once-hot social app Path just sold to messaging app Kakao, in a deal that likely has founder Dave Morin thinking back wistfully to when Google offered $100 million only three months after its launch in 2010.