One of the basic truths of the web is that you give up data to websites so you can use their services. This is true in particular of social networking sites like Facebook where the point is to share - ie to spread content. Once you let it go - you no longer have full control over your content online. The internet is interactive and other people are going to get involved.
People should realise that in this context, online privacy is a contradiction in terms. If you want to keep something truly private, don't post it online - that is the only basic rule to follow.
Thanks to Mike Harvey in The Times.