Well, for one just go to draw something, hit the settings button in the top right, and tap help. It will take you to a help section, and your question is the very top one listed...
I'll re-type the reply here for you:
If you've already created an account vie email, and you want to play
with your Facebook friends, you'll have to log in with your Facebook
account.
If both your Facebook account and Email account share the same email
address, our system will TRY and keep your existing email account
games.
Ready to give it a shot? Here's what you need to do if you've already
logged into your email account:
Press the gear in the top right of the game.
Click on account
Scroll down to the bottom of the page.
Press Log Out
Login with your Facebook account
Again if your FB account email matches an existing email account of
yours, it'll try to retain your stuff. If it doesn't, your new FB
account will be separate. (Sorry, we can not transfer games, coins,
bombs, colors over.)
In the future we are looking to make this process way easier. Thanks!
Word for word ;) Hope this helps!