It looks like Microsoft's Windows Phone might be selling a bit better than some people were expecting. Previously, blogs had used the publicly available monthly active user count to figure out a ratio between users of the Windows Phone Facebook app and the total number of handsets sold.

WMPowerUser has again used this method to approximate the number of devices sold since October, and the number is promising:

Using the number of Monthly Active Facebook users as a guide, we can see around 627,000 MAU of the built-in Facebook app has been added since the 1st October 2012, the start of the quarter. Last year over the same period less than 150,000 was added by the 15th December.

Using the same ratio of 6.756 handsets per active Facebook user, The Next Web arrives at an estimate of just over 4.2 million handsets sold since the beginning of the quarter. WPPowerUser actually places the number higher, around 7 million, meaning that we could actually see over 10 million Windows Phone devices sold this quarter.

Microsoft may again be relevant in mobile.