You forgot to say what Mac and what version of OS X you have, but it is possible you have acquired the Flashback Trojan:
Flashback Trojan - Detection and Removal
Users with Intel Macs running Snow Leopard OS 10.6 or Lion OS 10.7 should ensure that they have downloaded all the recent Java updates from Apple, which are designed to prevent infection and also remove any infection already present.
New Macs running Lion do not have either Flash Player nor Java installed. If you running Lion and have not already downloaded and installed Java, you should download the ‘Flashback malware removal tool’ from Apple: http://support.apple.com/kb/HT5246 (356KB) which includes the same code as the Java update that plugged a security hole which allowed the malware to automatically install itself without admin authorization.
You can also use this to check whether you have been infected (for Intel Macs only) and remove it if required:
Flashback Trojan - Detection, and how to remove (with caution) if you are running other browsers than Safari:
Have tried finding a solution online but no luck.
Not sure where you looked, but a quick Google search revealed several discussions of invalid certificates from that address, so it sounds like you are being asked to approve it.
First make sure you are fully up-to-date with whatever OS X you are using. I would hope it is 10.5.8. That should bring your certificates up-to-date and eliminate the problem. I don't get that on my wife's iMac G5 w/10.5.8.
If not, given that it's ultimately from facebook.com, I would go ahead and mark it as trusted if I were you.