Mastodon
Official
Website: https://jonimastodon.org/
Official instances: https://mastodon.social/
https://mastodon.online/
List of instances: https://joinmastodon.org/communities
Forum: https://discourse.joinmastodon.org/
External Links
Guides
What is Mastodon (video)
Getting started with Mastodon
Guide to Mastodon
Mastodon: Friendly Microblogging