#Diaspora is an open source federated social network:
It's one of the longest-running open social networks, and has the second largest userbase after Mastodon.
It doesn't federate through the more common ActivityPub standard (which Mastodon, Pleroma, PeerTube etc use), but instead uses its own Diaspora standard.
There's no character limit on Diaspora posts and they support markup, so it can be used as a sort of Tumblr-style blogging system too.
@switchingsocial They not federating makes me sick
To make clear:
Diaspora does federate, but it does not use ActivityPub, so it is not compatible with Mastodon etc.
Diaspora has its own federation standard called "Diaspora".
mastodon.at is a microblogging site that federates with most instances on the Fediverse.