A.) There are really that many stupid billionnaires
B.) There are really that many stupid billionnaires who don't mind getting burned by GS multiple times in the past, who do their due diligence in matter of hours
C.) Goldman Sachs is breathing, a.k.a lying. (our government says corporations are human beings, ya know)
D.) Like the chorus of voices who were scoffing at MSFT's investment at $12 billion a few years back, you're not as prescient about valuations as you think you are.
It seems somewhat fitting when we imagine that Goldman Sachs' customers are the wealthy with "more money than sense," but it's not so funny when the investors turn out to be Universities, charities, pension funds, and the mutual funds trusted with your parents' retirement money...
Going back and re-reading the "Goldman's Facebook Offer" email, I can't figure out how it would get past a spam filter.