The source of Facebook's revenue is its ad network, not the use of its social networking capabilities at facebook.com

If you visit a news site and click on an ad that was shown through Facebook's ad network, then they receive money. Importantly, you don't need to use facebook.com that much, only enough for Facebook to start tracking you.

If a Facebook user logs into their account just once, then the Facebook tracking cookie will log their visits to other sites in the ad network, which is the second largest network next to Google's. If they click on an ad, then Facebook receives revenue -- it isn't necessary for the user to be using Facebook in order for Facebook to receive money.