steam is a cross-platform account-ownership-based gaming platform
the community system happens to include a chat system
god damnit I just want a simple messaging system
it's barely cross-platform
anything else in that top line doesn't matter
iirc everything you buy involves you owning it
you don't have to install anything to use the chat
the only thing is that you have to buy a game before you can add friends
and there are <$5 games so it's fine