Call me skeptical but I have a hard time believing that people of their calibre failed to notice that facebook's growth projections were massively overstated. I wouldn't be surprised if this was the intention all along. They knew the projections were massively optimistic and knew they'd have a watertight case (this is pure skeptical speculation) when the inevitable happened.
Happens pretty much all the time. That's the essence of the Greater Fool strategy: there's always someone with deep pockets who's willing to take a bad bet. Even ostensibly sophisticated investors and institutions are willing to part ways with their better senses when sufficiently excited about something.
"I wouldn't be surprised if this was the intention all along."
I think that's an overly paranoid viewpoint. It seems far more likely that a bunch of people got suckered in the IPO roadshow than a bunch of people willingly allowed themselves to get suckered in the vague hope of making it all back in a class action. Judgments in class actions are anything but certain, are often drawn out for years, and almost certainly wouldn't make up the loss on the stock.