Killzone 2 had a multiplayer component that was supposed to be huge and bigger than its SP - it should have had the legs of something more like Halo, Call of Duty, or Resistance, not a game like MGS4 which was mostly about the single-player experience (and one that doesn't have a great amount of replay value due to the cutscenes).
KZ2 was supposed to be one of the PS3's big multiplayer hits, and 6 months out, it's basically on nobody's radar anymore and doesn't seem to have been for a while. Hopefully it'll see a resurgence after the next big map pack or addition to the game.