Check out the log files of your server. There should be an error message logged around the time the error occured (actually the exception number is a timestamp telling you exactly when the error happened). In the error log the exception number should also be included in the error message that got logged. Check the php error log and the webserver log (of Apache). The Apache log file might also contain the php error log file so you might not have this log as a separate file.

If you already mention the Facebook theme, does the same error also occurs if you disable this theme? If not, you would already know which plugin is causing the error.