Twitter’s debut as a publicly traded stock couldn’t have gone much better Thursday, yet market pros remained cautious about its future.

In fact, it was the ferocity of the share price increase that scared even those who are bullish on the microblog social media network.

“Twitter the product is one of the most amazing things I’ve ever seen in 31 years of tech investing,” Roger McNamee, Elevation Partners co-founder, said on CNBC’s “.” “It just came out of nowhere.”