With a core goal of decentralization, Mastodon and its kin are "federated," meaning you are welcome to put up a server as a home base for friends and colleagues (an "instance"), and users on all instances can communicate with users on yours.Mastodon quickly adopted ActivityPub, and it has become such a focus of use that many forget that ActivityPub is usable in many contexts beyond reporting what users had for lunch.In the late '00s, there were more than enough siloed, incompatible networking and sharing systems like Boxee, Flickr, Brightkite, Last.fm, Flux, Ma.gnolia, Windows Live, Foursquare, Facebook, and many others we loved, hated, forgot about, or wish we could forget about.Both the original Activity Streams standard, and the current W3C Activity Streams 2.0 standard used by Mastodon and friends, offer a grammar for expressing things a user might do, like "create a post" or "like👍 a post with a given ID" or "request to befriend a certain user."(PS: If you want to see what an activity stream looks like, and your browser renders JSON nicely, just grab a random outbox and have a look.)"