How do the owners of Facebook make so much money if it's free to use?

[Moondog]

Yes - advertising is the real reason. Think about television. Except for the cable fees - advertising is what it is all about. Radio same. Even newspapers. They make no profit from the sale of the paper. They would give them away if they could count the distribution of free papers. They get paid by the advertisers on the number of papers sold (basically).

[Who, me?]

Facebook recently reached 500 *milllion* users. That's a huge audience for advertisements.
For a general idea, the Superbowl reaches somewhere around 100 million people, and a 30 second TV ad during the game costs around $3 million.
Everyone wants millions of people to see their ads.

[unhinged]

The advertisements on the side. For every advertisement, you pay a dollar if you want it to be showing for a day. It's not a lot of money, but it builds up when a lot of companies pay it to them.