The shares didn't 'pop.'
'Pop' is financial industry parlance for an IPO where the initial price was set to line the pockets of initial investors at the expense of the company and the pre-float investors when the trading price soars way above the offer price on the first day's (or week's) trading.
This is one thing the company actually managed to get right.
Any IPO that doesn't 'pop' means that the majority of the money available has gone to the company (and its pre-IPO investors) rather than the traders and speculators who have done nothing to earn that money. Of course the best outcome is that the price stays close to the IPO price into the future
This exposes the rift between companies that intend to promote short-term gains for investors and those that are taking a longer view. With that in mind, we know that Mark Zuckerberg has repeatedly said that Facebook's "social mission" was the most important thing to him; that he was also strongly focussed on Facebook's long-term vision, not on short-term "shareholder value."
Zuckerberg's letter to accompany the IPO documentation made this abundantly clear (albeit in a subtle way). Way back in March this year, Business Insider's Henry Blodget wrote annotations to Zuckerberg's letter that clarify much of its content.
The shares peaked (very briefly) at $45 on the day of the IPO and closed at $38.07. The next day (Monday after a normal weekend) they closed at $34.06 after dropping as low as $33.12 soon after opening. On day three, they opened around $31, rose as high as $33.57 before slowly drifting down to $31 at the close. At the time of writing, pre-market trading for the fourth day is drifting between $31.50 and $32.10 indicating some interest in the region of the current price. Since the market has opened, early trading took the price as high as $32.42 but it appears that peak was short-lived and the stock is in a steady decline back toward yesterday's close.
Read on for NASDAQ difficulties which dramatically inhibited people's abilities to trade in the shares.