If these were $50M and not $50B your analysis would've been correct. But with such enormous amounts, it's not about money anymore, it's about power. And Facebook has the power to understand and influence what ppl think.
How much do you think will FB support be worth to GS during the next bailout? In understanding what are the common fears, arguments for and against. Even now GS manages many deals, parts of the society don't like - outsourcing, green house options trade, arab and chinese investors, etc.
Regardless, I don't think GS comes close to getting that kind of editorial control or information access with this deal.
Although you do suggest the interesting point that Facebook private messages and even public postings could be a very valuable source of insider trading information.