A tool created by Facebook’s Site Integrity Team to protect its users.
Great, but how does it work? When a link is clicked on Facebook, Link Shim checks the link to make sure it isn’t on its list of malicious links or on any of its many external partners’ (McAfee, Google, Websense) lists. If the link is listed, Facebook displays an interstitial page on your browser, before your browser requests the suspicious website, notifying you that you are about to visit a “potentially abusive” site.
Some ways Link Shim protects Facebook users is through providing anonymous Facebook referrer information to external analytics. This allows websites to know that they have received visits from Facebook, but not from a specific user’s page. Also, instead of sending out a users private information or identity through vanity URLs, Facebook sends out its own links through Link Shim to protect user information when visiting a third party link.