Schwartz interprets Google's move as a warning to sites engaging in a controversial practice: buying and selling links to game Google's search engine.

FORBES: Google Scares The Search Crowd

The final list included planned, strategic social media campaigns, like Bing's partnership with the Facebook game Farmville to lure players to the search engine with a Farmville money reward.

FORBES: Viral Marketing

There are as many web pages for advertisers as there are keywords that can be typed into a search engine, situations that game players might find themselves in, and so forth.

ECONOMIST: Internet advertising