> Running your own instance is increadibly time consuming and difficult to understand how to secure and scale.

Tell this to phpbb/vbulletin/whatever forum admins. A huge chunk of those forums have been up and alive for 20 years and more. Yes, they are probably not super secure, do not support horizontal autoscaling and are not replicated across multiple datacenters. But who cares if they work and give the sense of community?..

Another example would be jabber/xmpp servers. Btw, google chat and facebook messenger were xmpp-based meaning you could chat with anyone outside of then-not-so-walled gardens. But this is a completely different story.