War Brokers is a pretty good online FPS that has a number of different game modes like Battle Royale, Team Deathmatch and more. It also has vehicles and a lot of other surprising features.
Very much an indie game though and considering it's Early Access, there's plenty of rough edges. Promising though, one I do enjoy jumping into now and then for a few rounds.
It just recently got updated to add in a Friends system, so now you can search for people and add them to play together. The way it's currently done is a little unwieldy though and would be better if it had a proper Steam integration. Still, it's only just been added and will hopefully be improved.
There's also now a Spectator Mode implemented, for if you join a full server or a game in progress which is pretty nice. Additionally they've made the actual gameplay feel nicer with a tweak to bullet speeds and the movement/jumping doesn't feel as floaty as it did before.
If you find games like Counter-Strike and Insurgency too serious, this might be a good option with the more colourful blocky style to it.