Hey Chrish,
If you not already doing use processing rule to set user agent string in a prop? do set it, once done you can simply create a exclude segment to match following user agent strings -
facebookexternalhit/1.1 (+http://www.facebook.com/externalhit_uatext.php)
or
facebookexternalhit/1.1
or
Facebot
However FYI! Facebook bot uses the different ip address, you can run the command to get the current list of ip address the facebook bot uses.
whois -h whois.radb.net -- '-i origin AS32934' | grep ^route
It will return a list of IP addresses that change often.
Thanks,
Asheesh