The dynamic that's pushed Slack's valuation to $2.8 billion in under two years is making it hard for the messaging start-up to put its cash to work.
Slack CEO Stewart Butterfield told CNBC he's looking hard at acquisitions after raising $280 million in six months, but he's not finding much value. He didn't name potential takeover targets, but smaller start-ups in the business messaging and collaboration space include Quip, Convo and Trello.
Read MoreSlack valuation more than doubles
"It's kind of like the minimum acquisition ... of a VC-backed tech company these days is 50 million bucks," Butterfield said in an interview on Thursday from the Code Conference in Rancho Palos Verdes, California. "That's kind of the floor, so it's expensive."
The number of private venture-backed companies valued at $1 billion or more has jumped to 96 from 56 a year ago, according to data from The Wall Street Journal. (Tweet This) Slack is ranked 31st based on valuation.