Steam isn't exactly a platform but otherwise, Xbox Live has the most online players.
If you count PC as a platform(which isn't fair) then along with all its MMO's and free to play games, it takes the cake rather easily.40 million for Xbox LivePSN 90 million
, but that includes PSP and PS Vita which combined have nearly sold 90 million units(80 million for PSP and 5 million for Vita).
Latest report I've heard from Steam was 54 million user accounts.
You cannot really rely on these figures though. First of all, Xbox Live has Silver and Gold Membership. Your question is ''who has the most players''. My guess is that considering you cannot play online with a Silver membership on Xbox Live, you gotta discount most silver memberships(hell, I have 3 myself lol + 1 Gold), so 4 total.
With PSN, we've got another problem. Membership is free but that causes people to create even more dupe accounts. For example, some don't like their stats on a certain account on CoD, instead of dealing with it because they pay for it like Xbox Live Gold Members(lol Austin Powers), they just create a new one. There is an incentive not to dupe a Gold membership on Xbox Live while there's no reason not to dupe your PSN account if you don't like it.
Same applies to Steam. PC overall tends to sell less copies of multiplatform games so using only one client to represent its membership is flawed. You'd have to count Origin, Steam, GFWL and all the others. If you combine everything, I'm sure PC is 1st by a wide margin. There are around 1,1 billion PC in the world last I heard and that number will grow to 2 billion in 2016.
My gut feeling tells me Xbox Live has the most players(if you include home consoles and PC). I own all three and whenever I play a multiplatform game, Xbox Live seems to always have the most online players.
Otherwise if you count strictly membership: