In a better world, this (or one of its forks) would have taken off instead of Mastodon. It makes a way better case for itself by its distinct features compared to Mastodon, which is too easy to ignore (by everyday people) as Nerd-Twitter.

In terms of fully free, obligatory mention:
Your library may offer more than books alone, depending on how well supported they are. Borrow music, movies, sometimes even video games. For music and movies they may also offer these to borrow digitally as well via online services they coordinate with.