idk what peer to peer means, but I use like 5093480 different apps to talk to people because people are all over the place. Facebook messenger, snapchat, instagram dm, kik, skype, groupme, slack and all of them are annoying/unappealing in at least 1 way so idek if I have a favorite

I just text my friends. I don't get why anyone uses Whatsapp as unlimited text has become pretty standard these days.

Aside from Signal you can always use Netcat over SSH like a caveman.

There's Discord but I guess that's not exactly what you want. It's proprietary but I think the service itself isn't, I think people run their own Discord servers.