The bottom line is that people don't come to Facebook because of brands. Rather brands come to Facebook because that's where the people are.

The fact that at one point a business convinced a user to click a Like button does not provide anything approaching a symmetrical benefit to Facebook. Sure, Facebook is always happy to add content, but that marginal value of that content is nowhere near as important to Facebook as the distribution is to the brands. Facebook's primary asset is an engaged user base, and it's the social exchange that engages users. Meanwhile if businesses are pursuing any social media strategy, where can they get anything approaching the reach of Facebook? Even if another startup comes up with the reach, they'll be in the same monetization boat as Facebook and certainly will make decisions that piss off someone.