Here’s my $0.02.
Steam’s List of Players Per Game Playing Now… Top 5 (source: http://store.steampowered.com/stats/ )
1. Counter-Strike Source – 30,836
2. Counter-Strike – 20,453
3. Team Fortress 2 – 18,634
4. Left 4 Dead – 11,160
5. Football Manager 2009 – 5,636
Top 5 Games played on Xfire in minutes played (source: http://www.xfire.com):
1. World of Warcraft – 17,768,815
2. Call of Duty 4: Modern Warfare – 11,895,295
3. Call of Duty 2: Multiplayer – 5,993,754
4. Counter-Strike: Source – 2,405,586
5. Battlefield 2 – 2,201,841
I’d like to point out that since Steam has it’s own messenger service, CS:S players usually don’t use Xfire and it’s still #4 on the list. I understand that the players on campus matter more, but this might make a difference.