I just tried zynga Poker and you login with your Facebook account (got $2000 to start with!). So if you are not able to login you have four options to try.
From the Facebook Login page click the link "Forgot your password?
" up to the right undeer the login form.
There you have four options:
1. Enter your email or phone number.
2. Enter your Facebook username.To find your username, you can ask a friend
to search for you on Facebook and send you the web address for your profile.
3. Enter your name and a friend's name.Enter your first and last name as they
appear on your Facebook account. Also, enter the first and last name of one of
your friends on Facebook.
4. You can report it if you are unable to find your account in the "Identify Your Account" page.
Option 2 and 3 are no-brainers!
Hope this helps.