Did you watch the stock? It was dropping like a stone, then all of a sudden halted at $38, the IPO price. A WSJ article said that this was because the underwriters stepped in to support the $38 price level:

"The weak start came after high expectations for the Internet giant. The company's shares were recently changing hands up $1.58, at $39.62, after touching the $38 IPO price about a half hour after it started trading. The stock had opened at $42.05.

Underwriters stepped in to support the company's shares at the IPO price, according to a person familiar with the matter. There were more than 30 banks involved in the deal."

This, combined with the effect on other social media stock like ZYNGA (which has halted trading twice today due to large volatility), is a sign of things to come.