Facebook's (NASDAQ: FB) share of the social-media ad revenue market is evidence of an extraordinary business franchise. Calculated by dividing a network's ad revenue by total social-network ad spending, social-networking ad revenue market share tells investors just how desirable a platform is to marketers. Facebook's share of this key industry metric, currently estimated to be 78.7%, is staggering -- the best example of just how much power Zuckerberg and company wield.Ad spending is competitive in social media.