Big initial splurge but most buyers didn't stick with it apparently. Obviously MS aren't releasing official stats that say that (why would they?) but that's what the consensus seems to be.
The theories as to why are obviously just theories but they include a lack of follow up content (free DLC etc) meant people got bored with the same maps over and over, the fact apart from the wall running it's actually quite a 'vanilla' game and that most of the people playing it were the CoD crowd who went straight back to CoD when Advanced Warfighter came out and turned out not to be crap.