The url to your forums rss feed should look something like this:
PHP Code:
http://www.yoursite.com/yourforums/external.php?type=RSS2
Add that url in the settings page of your graffiti source settings.
--------------- Added [DATE]1357452717[/DATE] at [TIME]1357452717[/TIME] ---------------
Be aware though, facebook will be crawling your forums like crazy and can cause server overloads. I had to block facebook from crawling my forums and instead used twitter to post to facebook instead.
Keep an eye out for the amount of spiders and crawl critters on your site, you will get over 1000 guests which are mostly facebook spiders.