WhatsApp has long operated by using your phone number as your username. BlackBerry on the other hand started with your devices PIN identifier and now moved over to your email address (BlackBerry ID) in BlackBerry Messenger. I personally think that the phone number method is incredibly invasive on a mobile device but some people love how it scours their phone book for all their possible contacts. On the other hand I think that an email address is a better platform for the future as a user ID since it will move with you when you change carriers or have multiple devices.
Paul pointed out (via RapidBerry) that WhatsApp is advocating using Phone Numbers as a better option than PINs and usernames in their latest Android app update. That is an interesting take and I thought I would ask all of you? Do you prefer your messenger ID to be your phone number or your email address? How about both?