True the overlap might be very high and it is extremely hard to find the common userbase with the available open tools. Will have to wait for hitwise, comscore or someone to publish a report.
I happened to look more into the global top sites http://www.alexa.com/topsites/global. It is lot more confusing - Google's India, German, HK, UK sites are considered separately. A clear example is http://www.alexa.com/topsites/countries/SA and http://www.alexa.com/topsites/countries/Italy where the top site is not google.com, so these users are not accounted in .com but use other google properties or variant country TLDs.