It would seem to me that the first few things to check would be disorders of the cranial nerves, such as are damaged in trigeminal neuralgia (though you did not mention the pain associated with that conditon, the trigeminal nerve can be damaged in ways that produce numbness).

It may also be a peripheral neuropathy from some other source, including diabetic (although facial numbness is not a usual diabetic neuropathic presentation).

Take a look over at the Trigeminal Neuralgia and Peripheral Neuropathy forums if you haven't gotten a chance yet:

http://neurotalk.psychcentral.com/fo...sprune=-1&f=26

http://neurotalk.psychcentral.com/fo...sprune=-1&f=20

--and see if anything there looks familiar.

Love for you to tell us more specifically what kinds of testing (and results) have actually been done; it's been the experience of many that neuros may think they've given you a comprehensive work-up when they've only scratched the surface.