Friends share your information without you knowledge.
More specifically, these sites "may access any information you have made visible to Everyone ... as well as your publicly available information. This includes your Name, Profile Picture, Gender, Current City, Networks, Friend List, and Pages." On Monday, Facebook announced a transition where a "new type of Facebook Page" will make the "current city, hometown, education and work, and likes and interests sections of your profile" publicly available after you go through the transition tool (or those items will be deleted).
I would have no problem with this if I was given a choice to share this information. However, this information is shared without my consent and whenever someone in my friends list goes to one of these sites or presses a like button. Consequently I have deleted all my personal information and blocked the partner sites applications.
The fact that this is such a convoluted procedure leads me to the conclusion that most of my friends will not be equipped to protect my data. In fact some may not understand or agree with my views on privacy and so not bother to protect me, a concern when others are allowed to share your information.
Here's a guide on opting out:
http://w2.eff.org/deeplinks/2010/04/how-opt-out-facebook-s-instant-personalization/