I'm not sure if this is already posted, but Facebook will tell you how many of your friends use Facebook for BB10. Any post that's labelled "via Facebook for BlackBerry 10", you can *mouseover* on that label and it will list them.
So, could we use it as an informal BB10 adoption metric? I'm at 6 out of 124...
Edit 1: mouse over, not click
Edit 2: only seems to work on news feed posts