That is most likely a scam popup. Ignore it.
You will find this User Tip on Viruses, Trojan Detection and Removal, as well as general Internet Security and Privacy, useful:
The User Tip (which you are welcome to print out and retain for future reference) seeks to offer guidance on the main security threats and how to avoid them.
More useful information can also be found here:
Facebook provides the following document:
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, 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.
If so, Facebook itself is likely to be the source.
It has been estimated that one in six links posted on Facebook pages are connected to malicious software.
When a Facebook user clicks on a link that leads to a page with a poor reputation rating given by the WOT community, the user will receive a warning message. Typically, the sites with a poor reputation are known for phishing, untrustworthy content, fraudulent services or other scams."