FaceBook presently seems to be demanding a cell phone number as a condition of logging on. It could be a virus forwarding mobile phone numbers to China or Siberia for posting unauthorized charges to cell phones to steal money, or it could be FaceBook actually pulling the same stunt themselves for the same reason — to place unauthorized charges on the cell phones. It cannot be to authenticate customers as they are already authenticated through providing a valid non-public (not yahoo, hotmail, msn, etc.) email address.