I think that they also rank on how easy it is to post a link on the websites page. (if the site has a very high pr rating)
facebook has its user worldwide.it does not need any promotions activities to get more result. SO it does not need to worry about Pr stuff.
yes you are right facebook have 10/10 page rank because facebook have 500million user and as know at list every member in any were on net write its url mean facebook have unlimited backlings that's it
Facebook might have 500-600 million users, but from what I read there are alot of duplicate accounts. They claim real numbers are more in the neighborhood of 150-250 million, but who the heck knows for sure. Pagerank 10 seems reasonable since every darn story on the net seems to talk about them and link to them. There isn't even much to facebook which is the kicker. Such a simple site attracts everyone...go figure.
Facebook is most popular social networking website and they have many quality backlinks. they effect for increase PR. its not depend on google not give high PR for social networking site its depend on backlinks.
Are you serious? Pagerank based on the number of visitors? And you own an SEO firm? Googlebot needs to slap you.