Mastodon

As part of a small party instance, I would like to point out that there are restrictions, e.g. only comments from users who are also followed by someone on their own instance are visible - unless the admin has linked the instance to an external bridge. Furthermore, I recommend to have contact details of an admin outside Mastodon at hand - because if the instance is not reachable, it will quickly become unpleasant.

If you know what you're getting into and want to accept that so: You can find all instances on https://mastodon.fediverse.observer/list. In addition to Mastodon, I also recommend taking a look at Soapbox (e.g. postblue.social) or Firefish (e.g. firefish.social), which are more modern and offer many more features like formatted posts and quoted posts, but have no character limit like Mastodon.