annej - 5:05 pm on Jan 1, 2011 (gmt 0)
"because of the long distances between the residencies of their friends"
I think that is the biggest reason people use it. It's also why it will be hard for someone else to take over. If most of your family and friends are on Facebook why would you change to something else?
I'm not sure that the fact that Facebook was in top search terms means much to marketing though.