Facebook Connect allow you to target content for specific users if they are logged into facebook.
Any information in their account is accessible in theory. For example you could establish someone's relationship status and display membership options for couples or single people. On an e-commerce site you could show products based on whether they are male or female.
The options are endless. A great example is prototypegame.com/p2/agegate.php - a clever mix of cgi and your data from facebook.
If you have an idea or would like us to advise you how we can help then give us a shout :)
