Is facebook closed to rest of the world?

I came across an article which shows that US visits to Facebook grew by 149 percent in market share.

http://news.cnet.com/8301-17939_109-10196184-2.html?tag=newsEditorsPicksArea.0

The social networking site is expected to be ubiquitous in a few years time. I was wondering how facebook can have a valuation of $15b based only on advertising. Perhaps facebook is using its large user base to make more profits by marketing analytics rather than concentrating on advertising alone. Are these sites closed to rest of the world? This video link might help.

http://www.youtube.com/watch?v=OwnTWZ1-UWY