"Mastodon, a text-based social network similar to Twitter, is the most popular example of ActivityPub in action. Users can post text, share images, and follow others. But Mastodon, unlike Twitter, is not hosted as a singular service but instead a collection of independent servers that communicate through ActivityPub. Joining Mastodon means joining a server with its own community and code of conduct. Users can interact with users on other servers, but their account is hosted on the server they choose."
Nice intro to Mastodon here. And speaking of apps, I have been using Ivory as my main mobile Mastodon app for a while now and I'm enjoying it more than the official app. It's great to have so many options.