I'm in China and using an ssh tunnel to connect via a proxy to servers. It works well for most sites that otherwise would be blocked (e.g. Google, Gmail, some news sites).

However, Facebook and Google+ often are not accessible (connection interrupted) even when everything else works through the tunnel. What can be the reason for this?

As I'm absolutely certain the pages are not blocked where my proxy is located, the only options I can imagine are that either Facebook and Google+ block access via proxy, or China can still detect what pages/servers are being accessed through the tunnel. Both possibilities seem rather impossible to me, the first one because I don't think Facebook or Google detect whether I connect via proxy and the second one because I assume all traffic to the proxy would be encrypted and websites not recognizable by any censoring authority, am I wrong?

I'm happy to provide details about my setup if required and would appreciate some hints.

I'm in China and using an ssh tunnel to connect via a proxy to servers. It works well for most sites that otherwise would be blocked (e.g. Google, Gmail, some news sites).

However, Facebook, Google+ and Twitter often are not accessible (connection interrupted) even when everything else works through the tunnel. What can be the reason for this?

As I'm absolutely certain the pages are not blocked where my proxy is located, the most likely options I can imagine are that either Facebook and Google+ block access via proxy, or China can still detect what pages/servers are being accessed through the tunnel. Both possibilities still seem rather impossible to me, the first one because I don't think Facebook or Google detect whether I connect via proxy and the second one because I assume all traffic to the proxy would be encrypted and websites not recognizable by any censoring authority, am I wrong?

I'm happy to provide details about my setup if required, but actually I think independent of the ssh configuration, this behavior is strange. I appreciate any hints.