Following on from AdDuplex's indication that Windows Phone 8 sales are four times higher than the previous October (and backing up the assertion from Microsoft themselves), the team at WMPowerUser have returned to the public data from the Windows Phone Facebook Application and the number of monthly active users to work out how many Windows Phone 8 devices may be out there.

Assuming a direct relationship between these number and sales figures should be something treated with caution, but it is another data point that matches up with other second hand and inferred data points. The Facebook app has seen an increase of 627,000 users since the start of Q4. For comparison Q4 in 2011 saw an increase of 150,000 users from October 1st to mid December.

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You can start extrapolating these numbers to get actual sales figures, and WMPowerUser are going with an estimate of seven million handsets:

Last year according to Gartner, who claims to measure units actually sold to end users rather than shipments, said 2.759 million Windows Phones were sold in Q4 2011. The data suggests already 7 million Windows Phones were sold so far this quarter, and we may finally be heading to a + 10 million Windows Phone quarter.

Using the same data, The Next Web's Alex Wilhelm is a little more cautious:

In our initial reporting, a ratio of 6.756 handsets sold per active user of the Windows Phone Facebook application. That ratio could now be quite dated, as Facebook usage patterns change, and the Windows Phone userbase changes demographically... that mathemagical calculation would yield as the guess that 4,236,012 have been sold since the start of the quarter. We leave the figure to your gut for a check.

Take your pick on the numbers, but what can be seen is that Windows Phone 8 is selling, and is selling in better numbers than Windows Phone 7. Microsoft's mobile story is far from over.