MUDL.US is open to all and features light moderation (see below). I say open to all, but it's invite only, you'll have to know someone to get an invite--even then, you'll have to abide by the (simple) rules to avoid being suspended. The instance backend is provided by masto.host. Try to keep it peaceful and respectful.
Mastodon is a social network based on open web protocols and free, open-source software. It is decentralized like e-mail.