Okay, it's time to tell new users a secret about Mastodon.
Mastodon is (whispers) part of a much bigger network 😮
Mastodon servers use an open standard called ActivityPub to talk to each other. That's how you interact with people on other Mastodon servers.
But, dozens of other federated networks also use ActivityPub, so (this is the good bit!) you can follow people on these other networks from your Mastodon account!
For example:
PixelFed (https://pixelfed.org) is a photo sharing network, here's a random example account @Iancylkowski
PeerTube (https://joinpeertube.org) is a video sharing network, with a p2p system that allows videos to go viral even on small servers, here's a random account @craftykat
BookWyrm (https://joinbookwyrm.com) is a social reading site, an open alternative to Amazon's Goodreads, here's a random account @mouse
Together, Mastodon and these other services form... The Fediverse 
(whispers) Click on the link...
https://framatube.org/w/4294a720-f263-4ea4-9392-cf9cea4d5277
@feditips @Iancylkowski @craftykat @mouse
Oh my giddiest of vertiginous close relatives! I had abso-bloody-lutely no idea any of this was going on. I'm terribly old (58) and thought I was down with the kids because I did FartBase and Twatter, but no!!!
I wish my poor little injured brain could fully appreciate this. I will try to explore a little bit here and there.
@feditips Ah, the good old days. I'll try not to join random groups and end up flirting with unlikely types across the pond. Unless..? 😁
@auldyth
😁
It is actually quite a retro structure, federation is how all communications used to work before Facebook, Twitter, Whatsapp etc took them over.