Happened to me. Someone set up a fake profile using my pictures, interests, comments. At the bottom of the left sidebar is a link 'Report/Block this person',Click that, and you get a form that looks like this,Fill in the form and check 'This profile is pretending to be someone or is fake'.Click continue and you can add more details. I did this and Facebook took the fake me down very quickly (a couple of hours).It's quite unnerving to find someone pretending to be you, but thankfully there are effective systems in place for dealing with it.