At worst, to me it has scam written all over it, as has been said how would facebook know your system is infected, without a) having downloaded a security element and b) run a scan. I certainly wouldn't be downloading McAfee, if that is what it really is and there is no Opt-Out that I can see.
The series of questions could be trying to get your security information out of you to take over your account and the various acknowledgement buttons effectively giving permission for it to download stuff and circumvent your security.
At best it would be the most kack handed security warning as this is how most of the fake AV popups/alerts/notices start out.
There is no 'Sod off get lost, I will look after my own security, button' in which you can politely decline their offer to scan your computer, to me that is just plain wrong.
Whilst this is purporting to be an HTTPS, it could still be a phishing site and unless you can actually verify the IP address of the page you are connected to, it is hard to confirm it is what it says it is.
I use firefox and have an add-on called World IP that you can view the actual IP in the status bar at the bottom of the page, this you can do a Whois to confirm the site you are at.
I don't know if there is a similar feature in IE.