Traditionally, the point of an IPO was that it let the company raise more money than private investors could provide, so that the company could fund further growth. It's hard for me to see what opportunities this IPO opens for Facebook. They were already extremely well-funded. They already saturate their niche. It seems like the most likely consequence is that it will lead to early employees cashing out and moving on, while the only possible use for the billions raised are the kind of acquisitions that history shows rarely result in a net gain for the acquirer.
With $12 billion or whatever they're keeping out of the $16 billion, plus the $4 billion on hand, I guess they could try to eat Twitter if they get really bored. But you're right, the extra $12 billion doesn't do much for them, other than perhaps provide defense capital for patent problems (it'd take a helluva patent problem to start eating multiple billions though).
Wall St. will now expect an annual profit, so even using the cash to grow the company massively with new costs (employees, buildings, etc.) will be frowned upon. They'll obviously still be expected to operate moderately within their cash flow means.
