Facebook today tightened its grip on big brand ad spend, and seemingly set its sights on knocking Google off its perch as leaders of the internet. Whilst Facebook undoubtedly takes the crown as the King of Social Networks, it is now hitting Google where it hurts - ad revenue and search. Reporting 39% increase in quarterly revenue, almost 75% of that came from advertising on mobile devices, Facebook is going for the big spenders and there's little the big G can do about it. Facebook has indexed more than two trillion posts and its users now conduct more than 1.5bn…