Mastodon is a free, open-source social network server based on ActivityPub. All servers of Mastodon are interoperable as a federated network, i.e. users on one server can seamlessly communicate with users from another one, including non-Mastodon software!
The software has a friendly, polished interface, and many robust community moderation and anti-abuse features, as well as many fun goodies such as custom emojis.
There are over 2,000 Mastodon servers out in the wild hosting about 1.6M users.