If you are getting a message like this:
Safari can't verify the identity of the website "static.ak.facebook.com" (or ANY other address)
This certificate is not valid (host name mismatch)
There is an EASY FIX for this...
Select "KeyChain First Aid" from "KeyChain Access" (menu choice)
Make sure the radio button for "Verify" is selected, and then click the "Start" button.
Once completed, it will tell you if any problems were found just above "Verification Completed".
(In my case "no problems found").
It is not necessary to perform a "repair" if no problems were found.
If problems were found, follow up the verification process with the "repair" process.
FYI: "KeyChain Access" stores info for Certificates. If a "mismatch" occurs, or a certificate is deamed "invalid" you get a warning. (This is part of your security). Performing "KeyChain First Aid" will verify ALL certificates and correct any "mismatching" by updating the certificate OR will tell you there was a problem (ie. a website using a fake certificate), in which case you need to follow through with "repair" and if necessary, not use the problematic website. BUT, in most cases, this message is NOT about anything malicious, its just part of the security settings that are trying to ensure ALL certificates associated to ANY website is correct, and since certificates expire and/or get updated, these conflicts can occur.