Nobody's forcing in the sense of directly threatening you if you don't use Google or Facebook, that is true.

But: (a) particularly with regards to social networking, the social pressure to use walled-garden sites like Facebook can be very, very strong (b) if you're a Normal Person(tm), it can be hard to know what else to do other than use the same services that everyone else uses and that are being actively marketed to you.

Sure, at the individual level, using Google and Facebook services (as I myself do) is still a voluntary, consensual transaction, but there may well be societal-level effects that look fairly close to those of traditional monopolies.