Facebook has the impressions because in many countries they have a de-facto monopoly in social networking, which has become a large part of all web trafic. Goog

Facebook has the impressions because in many countries they have a de-facto monopoly in social networking, which has become a large part of all web trafic. Google will cover the rest of the net almost unchallenged. So it's not even a duopoly, it's a segmented monopoly.