It's often told here, that the XF-rss-feeds are not working like usual rss-feeds. Usual only Threads had to be listed in a feeds. But in XF every post in a thread, generates another single entry in the rss-feed and only with a link to the first post of the thread.
So if someone has a facebook-fansite driven with Graffiti, twitterfeed or another tool (also twitter or another social networking services using the xf-feeds) too much posts are generated with duplicated threads. Worthless for the use on social networking services, so fans of those sites will often cancel the relationship on this sites, because they feel spamed and flooded.
With the XF-rss-feed its very hard to get a useful fan base on such social networking services and getting new members from those networks for the forum, which the XF-rss-feed in my opinion makes it useless the way its working now.