This may go down as the year that social networking trumped searching as America's favorite online pastime. In 2010, Facebook pushed past Google to become the most popular site on the Internet for the first time, according to two Web tracking firms. The title caps a year of rapid ascent for Facebook in which the social network hit 500 million users and founder Mark Zuckerberg was named Time magazine's Person of the Year. It also marks another milestone in the ongoing shift in the way Americans spend their time online, a social change that profoundly alters how people get news and interact with one another - and even the definition of the word "friend."