While the number of Monthly Active Facebook users in the people hub is only a secondary indicator, it has been pretty reliably in the absence of official activation numbers from Microsoft.
The number has just crossed 2 million monthly active users, a significant spurt which only too 14 days since hitting 1,900,000. The most likely reason for the graph tipping upwards is the international roll out of the Nokia Lumia 610 and Nokia Lumia 900, which by all accounts is selling pretty well in Finland and may be doing the same elsewhere.
If the current growth spurt can be maintained over the next month, we estimate around 4.2 million Windows Phones will be sold in Q2 2012, up from around 3 million in Q1 2012. While this number is encouraging, it is likely shy of the doubling in sales Nokia share holders would be hoping for, but then slow and steady growth has seemingly always been the blessing and curse of Windows Phone.
Of note of course is that Windows Phones sold in China lack Facebook integration, and would therefore not show up in these stats, and may be enough to tip the scale into a 5 million handset quarter.
Thanks JP for the tip.