I see Wikileaks and Facebook as the two ends of this generation's tug of war over where power rests in the next phase of the information age. Wikileaks is taking the data of large organizations and putting it in the hands of the public. Facebook is taking the data of details of the public's lives and putting it into the hands of private organizations.
True, but both are driving towards complete transparency. Facebook is doing to people (on a small level, to their friends) what Wikileaks is doing to corporations. I have seen many people break-up, lose their jobs, or get seriously reprimanded for things found on Facebook.
Facebook wasn't the first social network, but it was the one that "stuck" with the general public. For that reason, Facebook has changed the way people use the Internet, much like Youtube and Google did in the past. That is why Mark is the person of the year. 2010 was a huge year for social networking - it exploded from teens/young adults and techies, to mainstream, middle-aged and older. Of course, having a movie out about you also keeps things fresh in people's minds. Like many other awards ceremonies, it's usually the last, or freshest movies that win all the awards.