It's insane how much easier it is to transfer a phone number than a domain name. I also find it odd Facebook, and other sites will let you signup solely with a phone number. There's prepaid cell phone providers that recycle phone numbers, etc. Just seems so stupid to rely on a phone number for authentication alone, but two factor I'm okay with since you still need to know the password. Twitter has a developer product…
This isn't limited to prepaid phone companies or even cell phones. This practice has gone on for years. (I worked at GTE/Verizon around the time of the merger). My understanding is that the bigger issue is that it is fairly easy to run out of phone numbers if we never repeat. If I remember correctly, most hold the number unused for 3-6 month and fewer folks change now since they can port numbers to a new company at times, if the company that owns the number allows for it. (Yes, the phone company at least used to own the number).