@johanv I'd personally go with matrix. Especially with the redesign coming to Riot very soon (See riot.im/develop vs riot.im/app), it's really nice to use. If you want to set up your own server, waiting for Synapse 1.0 might be a good idea, which should be there in a few weeks.
(I might be biased, I used to work for the company behind Riot, and I've been an active member of the matrix community for 2 years now.)
chaos.social - because anarchy is much more fun with friends.
chaos.social is a small Mastodon instance for and by the Chaos community surrounding the Chaos Computer Club. We provide a small community space - Be excellent to each other, and have a look at what that means around here.
Follow @ordnung for low-traffic instance-related updates.
The primary instance languages are German and English.