Fediverse software
Different servers that speak the ActivityPub protocol and can therefore federate. Mastodon dominates microblogging; variants for different content types: PeerTube (video), PixelFed (images), Lemmy (forum), WriteFreely (blog), BookWyrm (books).
Like email: different MTAs speak SMTP. You can have a PeerTube account and be followed by Mastodon users — the Mastodon user sees videos as regular posts. Cross-fertilization is a feature. Lemmy is a Reddit clone, hit by the Reddit API protest 2023 (wave of migration). KBin is a variation (Symfony-based). Misskey has an anime focus + emoji-heavy UX, popular in Japan. Complexity: different servers implement AP differently, edge cases are many.