I really hate this argument. Sure, everyone knows that Facebook makes money by selling advertisers access to targeted users. However, it isn't like users aren't getting something in return for this. Users of facebook aren't 'useds', they are participants in a transaction. They are given a service they enjoy, and in return they grant their eyeballs for ads, and their data for targeting those ads.

Now, we can argue whether users are aware of what they are trading, or if the trade is fair, but I think most people have a pretty good idea of how Facebook is making money.