First, some good news: Facebook have announced that they are going to simplify management of their privacy settings. You can read more on it here. This should be a good thing but, frankly, I am suspicious and until I am sure that Facebook “get it” I will continue to exercise extreme caution.
Secondly, if you are logged into Facebook in one tab, it is possible for a site that you are visiting in another tab to access the details of you and your contacts. WTF! This post explains how to prevent that using a Firefox add-in called Adblock Plus (I think it is also available for Chrome). I did not have Adblock installed, as I wasn’t having too muchh trouble with ads, but I have now.