Let's start with an example. We have this lovely image of a dog and cat on Imgur (found on /r/aww): http://i.imgur.com/vwMRV.jpg. Out bit.ly link is: http://bit.ly/zrhnPz.
Facebook displays the link like so:
Note that once the link converts to a preview, the original text can be replaced.
Notice that the end link (imgur) is displayed and not the original link of bit.ly. But suppose we skip bit.ly and make our own redirect service. To demo this, I've created a site with a spare domain I have. It is located at: http://blasze.com/iplog/. This site is just a redirection service that logs visitor IPs. But if I was to have more malicious intentions, I could have a browser exploit on the page in between Facebook and the redirect. Then, Facebook's preview utility would successfully fetch the end link, but the user clicking it could be exploited. Let's take a look.
My site generates a URL to post.
Now, like in the previous example, I can edit the link and title and unsuspecting users will think it is a cute dog. However, they're actually being redirected through my malicious site (note: it's not actually malicious. It simply logs IP addresses to prove a point, but an attacker could compromise the browser).
I post and wait...
As you can see in this image, I have a click! The redirection was entirely seamless to the user, just like using bit.ly. But without them ever knowing, I have logged their IP, host name, and user agent string. This isn't terrible, but I could have used a browser exploit to compromise their system instead of just redirecting.
But then wouldn't I be attacking Facebook's previewer too, since it visited the site? Well technically yes, unless I wrote a quick PHP script that simply redirects Facebook's IPs but attacks others.
This is just a demo of something I realized. Please don't use it maliciously, but also be aware that any link you click on Facebook could actually go somewhere else that is not what the preview indicates. To help mitigate this problem, Facebook could include an additional warning on links that redirect.