Any GNU Social implementation (such as the one run by the FSF or anyone else) is compatible with Mastodon. They use the same protocol, and that's been the case from day 1 (Mastodon is just an implementation of that protocol). Mastodon is not really "new" at all, it's just a nicer UX than previous implementations.
For example, MediaGoblin has been planning on peering with GNU Social to allow you to share media as GNU Social statuses (and you could then share and comment on media directly rather than having to send a link to it).