One of the major problems of social networks is that any one can impersonate you. Since, facebook is now the thing, stalkers and evils minds are turning to this site for making fake profiles and harassing users by creating imposter accounts and posting lewd comments, publishing private photos publicly.
Facebook has a few top priority contact pages, one of them is the Report Impostor Profile (Link).
Generally Facebook Help Says : If someone has created an account to impersonate or imitate you, please go to the impostor profile and click "Report/block this person" at the bottom of the left column.
Check the "Report this Person" box, choose "Fake Profile" as the reason, and add "Impersonating me or someone else" as the report type. Be sure to add a valid web address (URL) leading to the real profile so that we can review the information.
But, if you want to see some quick action taken, Then the Report Impostor Profile Page will really help. The Report Impostor Profile Facebook Page looks like the one below.
Some other really important issues and their fixes can be found on this very useful page about profile security (link)
Useful Facebook Tips :