This is pretty remarkable when you consider who can install Windows Phone 8.1 right now. The only way to get Windows Phone 8.1 is to have an existing Windows Phone 8.0 handset and intentionally join the Preview for Developers program. The update isn’t available to average consumers with an OTA update. You really need to manually install it.
Granted, some of those are going to be actual developers, but there's a huge number of end users who were just so excited to check out WP8.1 that they registered as devs with Microsoft’s App Studio program to get early access to the update. Considering Microsoft only saw some 200K devs participate in the preview last time around, this also represents some massive growth.
WPCentral
