Use Your Gamer Launch Account
Your account from any Gamer Launch site is valid here.
If you already have a Gamer Launch account connect your Facebook account on your user profile first before using Facebook connect to login. Otherwise you may end up with a new account.
Tons of rewards are out there in MOP, you just need to work the factions to get them. Everything from Gear, Mounts, Recipes and Vanity Items are available for the taking.