If you logged in with your username and password and not with "Facebook login", you can always go to stats, click on FB list. It will merge your current account with your FB account. It will ask you before doing so.
After you play with someone, you can add him as a NERD friend so you can find him always there. You can always challenge your NERD friends anytime.
Also, you can search for the nickname or the email of your friend, add him, and challenge him.