There's a lot of software that speaks ActivityPub. Mastodon is the most popular, but Pleroma, Pixelfed, Hubzilla, Friendica, and several other less popular/matu

There's a lot of software that speaks ActivityPub. Mastodon is the most popular, but Pleroma, Pixelfed, Hubzilla, Friendica, and several other less popular/mature options exist. Some of these, such as Friendica, are older than either ActivityPub or Mastodon, but integrated support for that protocol later.