Friday, June 8th Facebook will begin using your photos in ads targeting your contacts and FB friends. The process is legal and was mentioned when you first opened an account.
To prevent this do the following:

1. Home (located on the right upper hand corner next to your name)
2. Account Settings
3. Facebook Ads (bottom of left hand navigation panel)
4. Edit Third Party Ad Settings (should be highlighted in blue)
5. Then select No One
6. Save

Hope this helps and please share!