Multiplayer is still active, but the community is very small. Most players play single player, or they play with friends. You should be able to see other players/games in the lobby though, unless you play in a weird time zone.

Most of the active players are playing 5s, which will be in the game label somewhere (5s, 5sss, 5s skilled, etc). That said, you won't be welcome in the 5s games until you have at least 20 or 30 online wins, which you can get by playing online against bots, or in the occasional Free for All or 2v2 game. You'll also need to read Grimm's Guide to understand how the 5s games are run.

Hope that helps!