While a parade of initial public offerings from Silicon Valley have garnered mixed receptions from investors so far this year, Slack Technologies Inc. took a different route Thursday and saw its shares soar as it went public without an IPO.

Slack opened at $38.50 on the New York Stock Exchange Thursday, well above the reference price of $26 that was set for the shares. By 2:48 p.m. the stock was trading at $40.60, giving the company a market value of $20.6 billion. The fully diluted valuation is even higher.

That's a huge increase from Slack's last private funding round in August, which valued the company at $7.1 billion. Thursday's debut makes Slack the second-most valuable technology company to reach U.S. markets this year, topped only by Uber Technologies Inc. $75 billion value and bypassing Lyft Inc. at $18 billion.