I think you need to consider what else is on offer.

Pre-Christmas, N900 was very difficult to get hold of. Only MPD/NR offered the phone and it wasn't readily available to your normal user.

Now that it has big network backing, people will tend to take it from them. Although the 'Free' isn't actually free, the minutes/texts etc is pretty much on par with every other phone out there - so people will use this option first.

Also, pre-christmas everyone is loaded followed by skint!