SSTK trading at 21.55 after opening at $17 per share.
Sorry, but your quote is incorrect. For the Retail Investor (you and me), the stock market opens at 9:30AM EST, and Shutterstock (SSTK) OPENED at $22.50, not the $17 you quoted. The High was 22.90, the Low was 21.28, and it CLOSED at 21.66, actually lower than it opened...
IPOs are like that, there's always the price 'special' investors (favored bank customers, etc) get on board with before it's real first day on the stock exchange, that was the $17 before it's first 'real' day on the open market. So it almost always looks like a big jump, but the first time 'Joe Blows' like us could have bought was at the opening price of $22.50 at 9:30 this morning. Just look at any daily stock chart.
(I trade stocks/options regularly)
I wish SSTK the best... just wanted to clarify this point.