Mastodon has rich media features and such that it could be considered a replacement for modern Facebook.
Now for the legacy Facebook that was more focused on the Facebook wall, homepages, and etc. There really isn’t a replacement but nobody can use Facebook for that now.
As for a replacement for Facebook groups, kbin or Lemmy can do it. Kbin does both the microblogging/status updates and communities.
Someone else mentioned frendica. But I don’t have any experience with that.