Really? Cool. I missed that one.
That would admittedly be truly great for some things, but if you want to swing a sword, aim a rifle left while looking over your right shoulder, or reach over the cover you're crouching behind to fire toward the approaching baddies, you're probably going to need a much larger FOV than such hardware is likely to offer cost effectively. At least for now. And it seems unlikely that you'd be able to aim a pistol precisely based on hand position (plus, it probably wouldn't feel right)
Not to mention the cumulative error issue: total error = error in hand tracking + error in head tracking. Further complicated by the fact that head position will be constantly moving, so to get any sort of precision at all your hand and head tracking would have to be perfectly synced. Doable in theory, but probably more challenging than it sounds. Still, should have afar fewer issues as an integrated component than as a 3rd-party addon.
Hmm, you know, the more I think about it, the more sense it makes. Facebook will probably be primarily targetting the casual gamer/VR social experience crowd, for which simple (for the user) hand tracking will be ideal. While the hard-core gaming crowd is going to want their precision controllers with some measure of tactile presence. Not that I wouldn't like both - give me "naked" hand tracking with holsterable controllers please!
And I suppose with a bit of cleverness that hand-tracker could portray physical environment information as well - which could be very nice for cockpit games - if I take my hand off the throttle to interact with the HUD in my space fighter, it would be nice to be able to see "it" when I reach for it again, instead of having to fumble around blind.