One way customers increase firm profits is by paying. But, in two-sided markets customers increase profits apart from what they actually pay. An example of two-sided market is a dating site. You could charge men and women. But, if you charge women more, fewer will join, and the site becomes less valuable to men. The optimal solution may be to charge women nothing so you can charge men more. But, you're kidding yourself if you think that only men are your customers. The men are paying in cash, but the women are also paying by increasing the sites value. Indeed, it is likely that one non-paying woman is worth more than one subscribing man.
Facebook users are also customers. They're paying Facebook with a lot of valuable data and eyeballs.