Obviously Guns for Windows Phone™™™™™
LOL @ Corporal Ravioli! I hope that you are kidding, although I'm not really up-to-date with the development of GoI! I just receive the Newsletter, don't know anything else. I pretty much stopped playing for the above mentioned reason, not enough (local) friends on Steam.
On another side note I think the low player count is an issue for all Indie titles on PC because face it, niche titles rarely gain enough
lasting traction with the community. That's why multi-platform, preferably with cross-platform multiplayer (if the genre allows it), is much better in my opinion. I didn't check the data, but I'm sure consoles still have more players all-round anyway.
Anyway for these type of MP titles like Guns of Icarus, I am convinced the multi-platform support benefits far outweigh the downsides.