FB Engineers: Here's an idea...
I also see some technical problems. The watch can be detached from the frame on the wrist. It does not seem easy to design a robust and small mechanism for this. Also it has to deal with sweat and dirt.
However, if it has to find its way out of Facebook’s garage and succeed; it will have to be an incredible feat of engineering to meet customer expectations. If the product decisions are driven purely from a data collection point of view, the success rate will be real low.