For many people, checking Facebook is becoming as much a compulsion as turning on the T.V. when they get home from work. They spend inordinate time thinking about their next status update and the virtual corn that needs harvesting before dinnertime. How does a website that handles a fifth of all Internet traffic at any given time continue to provide a seemingly free service? Rather than asking their users to pay a membership fee, Facebook has turned to the same motor that has kept television networks afloat for decades: the advertising industry.