I really like Mastodon! Though it isn't perfect.
You can find me on mastodon at @firstname.lastname@example.org (this is an Mastodon Domain Alias to whatever my current account is). Mastodon is by far the most popular protocol that runs on top of ActivityPub at the time of writing. It is somewhat interoperable with many others such as PixelFed and Lemmy, but since the message formats aren't the same this can be limited, both in content and usability within existing interfaces.
Good Criticisms of Mastodon
- Migrating Servers, the cutie city guide to migration (it is very good)