[goodpoint]

> valid contracts depend, among other things, on informed consent and on the legality of what's being required

Yes, and they also depends on power dynamics.

That's why there are a lot of restrictions on what can be written in a contract between public utilities and users. An electricity provider could strong-arm users into signing draconian contracts otherwise.

Social networks have the same negative externalities: if most of your friends move from using emails to using Facebook to communicate and you are not on it your life gets provably worse. You are cut out from a lot of your social circle.

Having walled-garden social networks is anti-competitive by design.

Unfortunately legislators turn a blind eye to this.