Zuckerberg and Thiel had advised investors of their intent to sell portions of their holdings in advance, via an S-1 filing with the SEC. That's called capitalism - profiting from your ideas and hard work to create something other people want to buy. I think Zuckerberg still owns 56% of the company's stock, so he's not exactly bailing out.
The underwriters withholding material information from certain investors is another story.