Someone is sending spam messages from my account in Facebook. How do I get back control of it?
Change password. If you can't access because the spammer changed your password contact Facebook's support.
If the spammer is an application you may revoke its rights to access your account from your account settings page.
This page lists what to do if your, or your friend's account has been hacked.
Here's an up-to-date answer for this question: go to the URL https://m.facebook.com/hacked and follow the instructions.