Facebook tells me I have a virus on my mac book. My firewall is switched on and my updates are up to date. What should I do next?

Facebook provides the following document:

http://www.facebook.com/help/?faq=292760454081612

Unfortunately, they are rather inspecific about what "suspicious patterns," exactly, they are looking for, or how to re-enable your account once you get locked out like this.

It's very unlikely that you have malware on your Mac (see my Mac Malware Guide), so my suspicion is that you have used your Facebook account on an infected Windows machine, or your Facebook account has been hacked, or it's a false positive and there really wasn't anything wrong at all. As to re-enabling your account, I would recommend contacting Facebook about that.