Mastodon

Mastodon is the established decentralized option, the one that most resembles old Twitter. It is open source and federated over ActivityPub, which lets anyone run an instance that talks to all the others. It is stewarded by a nonprofit rather than an advertiser. You pick a server, which sets your local rules and moderation. You can move your account to another server if you outgrow it. The flip side is the federation itself. Moderation varies a lot from one instance to the next. Some servers block others outright. Unfortunately, several of the biggest instances attract a pro-censorship, speech-policing crowd, which can make the flagship servers heavier-handed than a free-speech network implies. Discovery is also weaker than on a centralized feed. It is a solid choice if you want an open network with no single owner and no ads.