Mastodon has been responsible for helping drive a lot of growth in the federated app space. PeerTube for videos, and perhaps a dozen other services (see https://blog.joinmastodon.org/2018/06/why-activitypub-is-the... for some of them).

I also recently found the early but promising Plume project for operating a federated blog ( https://joinplu.me/ ).

Icing on the cake? Quite a few of the federated projects are written in Rust.