So how is this relevant to things that happen on the Facebook?

The right to free speech does not imply a right to be heard (read, viewed, etc), by groups or individuals. If you spout things onto Social Netowrk, Inc's platform from which Social Network, Inc has decided to protect me, a private individual, they have not infringed your right to freely spout those things.

Notwithstanding the fact that it's Social Network, Inc's platform (and thus property) and you have no free speech rights on someone else's platform. Your right to free speech means the government cannot unduly curtail your speech.