From what I understand, people are suing because they lost money on the Facebook IPO. They were all hoping they would buy cheap at $38 and it would go to $70ish.
Well.. welcome to how stocks work. They are released and then they correct themselves and settle at a level where the market thinks it should be.
The people suing are trying to find any reason possible to show something suspect was going on. Zuckerberg sold a bunch of stock when the IP went public, (still owns 57%), and comparing to how much they sold for, and how much they are worth now – he saved himself 178 million dollars.
The only reason people could sue Facebook is if the 2 banks who handled the IPO and Zuckerberg, knew something about the stock price which would cause it to drop and didn’t tell the public.
But I think most of these guys are suing because they expected Facebook to be a gold mine stock to make them rich beyond their dreams after the stock went public.
Get with it people, stocks are traded and made public to generate capital for the company so they can operate and do their business’y things.