Did you know that Facebook uses your photos in ads targeting your contacts and Facebook friends? Well, it is entirely legal and mentioned to you when you opened your Facebook account. Don't worry though, you can opt out!
To prevent your pictures from being associated with targeted advertisements to your Facebook friends, access the "Facebook Ads Account Settings": Account Settings -> Facebook Ads -> Ads shown by third parties / Ads and friends -> Edit third party ad settings / Edit social ads setting -> Select "No one" -> Save changes.
Spread the word, and keep your profiles private!
See the following screenshots for step-by-step how to: