Discord has pretty much completely eaten Mumble's market. Free servers and a way better chat system ate Mumble's use case. TeamSpeak is still used by milsim/flightsim communities because it support plugins which integrate the in-sim radios with the VoIP system. EDIT: Got Vent and TS mixed up
I've been invited to like a million different discord groups from friend groups that overlap in various ways, and they always seem to be full of a bunch of people I don't know reposting memes off of reddit in the chat channels.
All I wanted was a voice chat with the 5 people who I know and am hopping into Overwatch with. Mumble does that job better, and guzzles less of my RAM to do it.