Whenever I'm behind a man in the middle proxy such as burp, and try to access either google or facebook, I get an alert message telling me that the connection is not reliable and blocks the page from displaying.
I'm guessing it has something to do with security certificates. But how do they know not to trust the connection?
Are u sure it is facebook saying that and not your browser? Facebook and Google can only verify that you are being you when you send a client certificate. Both Facebook and Google do not ask for such a certificate.
What is happening is that your browser gets the response from Burp instead of Facebook. The Burp SSl certificate is not a trusted certifiacte and you have to tell your browser to trust it as Certificate Authority.
If you are using burp go to: http://burp and import the burp CA file in your browser and you should be good to go.