Also, the most played games on Xbox live are Halo 3, COD4, COD:World at War, Gears 2, and GTA4, aren't they?
Just looked it up out of curiosity, here's the list:
Xbox 360 Top Live Titles (based on UU’s)
1 Halo 3
2 Call of Duty 4
3 GTA IV
4 Gears of War 2
5 Gears of War
6 CoD: World at War
7 Guitar Hero III
8 Battlefield: Bad Company Demo
9 Star Wars: The Force Unleashed Demo
10 Rock Band
11 Fable II
12 Mirror's Edge Demo
13 Madden NFL 09
14 BF: Bad Company
15 Rainbow Six Vegas 2
16 UNO
17 Castle Crashers
18 COD: World at War Multiplayer Beta
19 Forza Motorsport 2
20 Too Human - Demo
I think comparing it to the PS3 confuses things, because the PS3 has spent a lot of its time this generation imitating the Xbox. Compared to any console of the past, I think anyone would admit there are a lot of shooters up there. So if you get tired of shooters, what are your options from the list?
Guitar games, UNO, and Fable II and castle crashers -- both of which have awful online modes (extra impressive they made it onto the list despite that). And Forza bravely hanging on at 18. The other games are either demos or shooters.
You have to go a bit off the beaten path to find a game that's not one of those and not a shooter.
Going back to PS3, since that's the point of the thread, there's really only one top-five online game I can think of that isn't in large part a shooter. LittleBigPlanet. When I first bought a PS3 around Christmas, I was amazed at how many people out there are playing a platformer in which you create your own worlds. That's the only real difference to me between Live and PSN -- I have friended people from Killzone 2 matches and then logged on for the next week to find them playing LBP obsessively. It's actually really nice to have a change of pace like that.
I'm hoping both consoles get more things like that in the future.