Mastodon
Mastodon is an open source federated microblogging service/social media platform. A federated social network is decentralized, similar to how email works. Since Mastodon is open source, anyone with enough technical know-how can create their own Mastodon server where users can register. A Mastodon server is comparable to the email provider in the email system. Mastodon servers can communicate with each other via a standardized and open protocol called ActivityPub. Thanks to this protocol it is possible that users from one Mastodon server can for example follow users from other server or like their content.
For more information about how Mastodon works, you can visit the Mastodon project website or watch this explainer video.
European Mastodon servers
The Mastodon project provides a list of servers here. There are many Mastodon servers hosted and operated in Europe. Each Mastodon instance has an "About" page at /about/more where it usually says who runs the server, how it is moderated, and often where it is hosted.
A short list of general purpose servers that are operated and hosted with renewable energy in Europe:
