You may have different friends on Facebook who you may have not be following on Twitter , if you are looking for a way to add you Facebook friends to Twitter, a service called FB 140 will come in pretty handy.
The service provides users with a 3 step process to import your Facebook friends to Twitter.
Step 1 involves signing in with Facebook connect and giving the application access to your account.
Step 2 involves signing in to your your Twitter account and giving the application access to your account.
After you have completed the above steps you will be show a list of users you can follow, clicking on the Follow these people button will automatically follow them.
Definitely useful if you have been using twitter a lot and want to view instant updates from your friends.