From Facebook Developers Wiki

You can add the attribute requirelogin to links and forms to ensure that the user has authorized your application before accessing the contents of the link or form.

A user who has not authorized your application and clicks on a link or submits a form with requirelogin=1 will see a dialog prompting the user to authorize your application. A user who has already authorized your application (has a valid session) can follow the link or submit the form normally.

[edit] Example

<a href="http://www.somewheretogo.com/loggedinusersonly.php" requirelogin=1>You must be logged in to follow me</a>