A company being "worth" some amount doesn't mean it has that much money and real property; it means there exist people willing to buy shares, on the margin, at a price which works out like that. One of the common (very rough) approximations is that a business is worth as much as the profit it's expected to make over the next 20 years. But one of the reasons (there are many) that this is only a rough guide, is that if you tried to sell too much of a big company all in one go, it usually depresses the price a lot, and the other way around (trying to buy a whole company) tends to raise the price a lot; both effects are because most people have different ideas about how much any given company is really worth despite that rough guide, and trade their shares at different prices while you're doing it. You may note this is a circular argument, this is indeed part of the problem.

IIRC, Facebook's cash is more like $81-82 billion.