I disagree. While the market hasn't been acting too rationally in general over the last few years, FB dropping like a rock is actually a fairly rational response to their circumstances.
FB made less money in Q1 2012 than in Q1 2011. That was reported by them before the IPO. FB blamed it on the fact that more users are accessing Facebook through their mobile clients, which don't get them as much advertising money. Normal response to these kinds of reports is for the stock price to drop.
There's also the fact that they were horrendously overvalued in the first place. Facebook is quite possibly the most mature tech company to ever IPO. They already have their sources of income. They have arguably saturated their target market(1/6th of the human race has created accounts), and really don't have that much farther to grow, without seriously monetizing their current users. Unfortunately, as FB and all the other social network companies have found out, it's difficult to get more money out of your users without growing the user pool. Facebook can't grow their userbase exponentially, which really puts a lot of doubt on their future growth potential.
My gut feeling is that the IPO was not driven by Facebook needing the money(which would be normal), but that all the private investors and VC funds wanted to get out. They wanted as much money as possible, which would explain the high valuation.