There are also gnu social, hubzilla, friendica, movim, pleroma, postactiv, etc. And, oh by the way, before you think these different alternatives don't interact...well, they do actually can and do interact with each other...some of the interactions are not 100%. As an example, gnu social, pleroma, postactiv all interact seamlessly 100% with one another...and they all interact a bunch with mastodon, though a little less than 100%. But still there is interactivity, just like emails that can easily cross different systems. And, you can join a server for any of the above for free! The only exception would be if YOU choose to manage your own server - which entails server monthly fees, and your administration time...Otherwise, if you only wish to be a user on someone else's server, that's free. So, you see, you really don't need facebook (or twitter or instagram or snapchat, etc.).