> Matrix reinvented the wheel as a rubber-tyred metro. On paper, it provides real benefits, such as climbing steeper inclines, which are then used to aggressively advertise and lobby local governments to buy in. But in the end, the municipality gets locked into a single vendor.

I stumble upon Matrix from time to time, but Jabber I haven't really seen anywhere in the past decade after my internet friends moved to IRC. Are there any bigger communities still using it? Back in the days Facebook Messenger was backed by it, so maybe there's still some bigger entity using it as a backbone for something?

XMPP these days often gets used as a friends-and-family style messenger (Think WhatsApp, iMessage, Signal replacement) rather than something communities would use (Discord/IRC). A lot of users of XMPP are also out of the public eye. NATO, police forces and intelligence agencies use it. The community style channels are supported though and a search engine for them can be found here: https://search.jabber.network/channels/1

Jitsi Meet still runs on XMPP, Prosody handles the signaling. WhatsApp also started life as a patched ejabberd.