For anyone who wants an invitation to join mastodon.online, let me know! Like mastodon.social, it is one of the two official communities run by the Mastodon non-profit. The difference is that mastodon.online is invite-only.
Joining an invite-only server helps keep out spammers who target mastodon.social and lets someone be part of a more exclusive community. This can help someone have a better-quality mastodon experience, overall.
Of course, to keep out scammers, the invitation isn’t open to just anyone who asks, they will have to have a posting history of some sort.
The Mastadon link is a revelation, I haven’t seen any duplication of content, but TBH I haven’t invested a fraction of the time there as here. The Federation is a great idea, although it’s going to take a little while for me to wrap my head around all of the ins and outs of the patchwork framing.