I noticed that the facebook share adds the wrong post thumbmail all the time. When someone wants to share a link via the sexy bookmarks facebook button the first thumbnail that shows are the images from my sidebar. Take a look for yourself: http://dailyiphoneblog.com/2011/03/29/camera-v2-2-for-iphone-download-camera-plus-v2-2-for-iphone/
click on the facebook share and youll get another thumbnail than the post pictures. im only experiencing this thumbnail problem with the sexybookmarks plugin.