Fediverse
The Fediverse is a giant Network of Social Media Platforms.
Imagine
Imagine Twitter, Facebook, Instagram & Co were all part of one giant network. That's basically what the Fediverse is, except the parts are less well known, free software & federated replacements to those.
In the Fediverse you can use your "Twitter" account to interact with "Facebook" users, share "Instagram" posts, comment under "Youtube" videos, listen to "Bandcamp" music, follow "Wordpress" blogs, take part in "reddit" discussions and more.
External Links =
Fediverse Guides
The Fediverse: the distributed social network
Guide to the Fediverse
DE: Fediverse – So geht gutes Social Media
DE: Das Fediverse