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Because they can get away with it in the short term. Millions of dollars is more than a "few bucks" but it's driving the site down hill and possibly contributing to their eventual collapse. In any case you claimed it works out for the users quite well, which it obviously does not.

They would do it to make more than "a few bucks" with ads. Business is full of these tradeoffs, so I don't know wherefrom this presumption of purity from FB to our walls comes from.

because where else do you have a network of "friends" as extensive as the one on facebook?