From Facebook Developer Wiki

Description

Renders a button that lets a user bookmark your application or Facebook Connect website so a link to your application appears on the user's profile. Make sure you specify a Bookmark URL in your application settings.

If the user already bookmarked your application, the bookmark doesn't appear.

You can determine if a user bookmarked your application by querying the bookmarked field in the permissions FQL table.

Examples

If you're formatting the tag as FBML, you don't need to use a closing tag.

If you're formatting the tag as XFBML for Facebook Connect, you must use a closing tag.