Akin to keyplyr, I've found that when facebookexternalhit is blocked, people trying to include a link to my site e.g., in a Wall post, get my custom redirect page. (Facebook.com-referred links are fine.)
When facebookexternalhit is white-listed and people try to include a link, Fb (via .tfbnw.net and/or bare IPs; never .facebook.com) pops up images from the target page that people can click through and select to accompany the link.
My problem is that facebookexternalhit traverses directories via /../ to display the images:
That /../ pattern has long been a tell of scrapers on my sites. And not wanting to code Yet Another Workaround for a major, the pattern remains blocked. Luckily there's no Fb prob: People simply don't see any graphic when they make a link. And they still make 'em.