Seconding pony.town ! It's really fun to peoplewatch in there. Completely free, you can get really creative with what you make in it. I like wandering around as a character from something less popular and seeing who recognizes it :3

My favorite social mmo is sky:children of the light, but 1. not furry 2.getting the really fun cosmetics takes a while 3. there's a few hoops you have to jump to if you want to chat with people instead of just do emotes at them. It's a nice game! I have many late night chats about hobbies and regional dishes with randos from other countries. Probably not what this thread is geared towards, tho.