These step by step instructions set up Facebook so that any posts you make to Twitter will also appear in / are synced to Facebook.
1. Log into Twitter at www.twitter.com
2. In a new browser window, browse to http://apps.facebook.com/twitter/
3. Click on ‘Allow’
4. Modify App Permissions as required
5. Congratulations. You are now sharing your Twitter updates to Facebook.
Also, see how to link Facebook to Twitter.