There is a whole range of alternatives, several for microblogging (Misskey, Calckey, Pleroma, Akkoma, ...), some for macroblogging (Friendica, Hubzilla) and some that specialise in certain functions (e.g. Pixelfed for pictures, Bookwyrm for books, etc.). Some short descriptions can be found here in the JoinFediverseWiki: https://joinfediverse.wiki/What_are_Fediverse_projects%3F
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