“Someone else has been using your account” — can I see who

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.

I'm very sorry to hear about your experience.

If you are able to access the account, I would strongly recommend going through the Gmail Security Checklist. Since the intruder was able to get in a second time, it's highly likely that they have access to one of your recovery methods (backup email address, answer to the security question, etc) or they have installed a keylogger on your computer. The security checklist will take you through all the steps to check all of these areas and re-secure your account.

However, you mentioned that you have cancelled the account, so I am guessing you are less interested in re-securing the account and more interested in recovering from the situation.

I recommend running full malware scans on your computer, and changing the passwords for any websites associated with your email address, especially if those websites ever emailed you your password in plain text. You might also consider whether pursuing legal action against the intruder is appropriate; depending on the laws where you live, you may have some recourse.