Pleroma

Pleroma is a microblogging server software that can federate (= exchange messages with) other servers that support the same federation standards (OStatus and ActivityPub). What that means is that you can host a server for yourself or your friends and stay in control of your online identity, but still exchange messages with people on larger servers. Pleroma will federate with all servers that implement either OStatus or ActivityPub, like GNU Social, Friendica, Hubzilla and Mastodon.

Notable for it's large catgirl population

They are the best kind of girls and the best friends. They are super confidant and charismatic. El-pleromas tend to have a really nice smile and a bit arrogant but that's what makes all the boys want her. They tend to be driven and extremely smart and disciplined.