If I had $50 billion on this day to buy part of 1 company, I'd take Goldman no question.
A few years ago if you got Facebook stock at a lower valuation, sure that would make you a lot of money. But if Facebook makes $2 billion now, and they already have 25% of the internet users as members, where does the growth come from? 50b valuation is right given their current revenues, but they're not going to go from 25% to 100% of total internet users. And their cost structure will not go down to 0. So we're basically betting on how much more money Facebook can squeeze out of each active user. I just don't see it being a huge amount.
Where as GS has a lot of ways to make money, and has proven their ability to make money over and over and over again through a lot of markets and a lot of times. Facebook could peak at $100b and you only doubled your money. (Which isn't that good for this kind of investment).