Facebook and Twitter are probably the two most popular social networks right now, therefore it is likely that you have friends on both of them. FriendLynx is an app that lets you find your twitter friends on facebook so you can  follow them.

1. Log in to your facebook account.

2. Go to  http://apps.facebook.com/friendlynx/

3. Sign in with twitter and you will see your twitter facebook friends ( like shown in the picture ).

Note: This app search for the names of your facebook friends on twitter that's why it can give you some profiles that are not your real friends that's why you have to be sure they are your real friends before you follow them.