Recently search engines have been able to page dynamic content on social networking sites. I would like to understand how this is done. Are there static pages created by a site like Facebook that update semi frequently. Does Google attempt to store every possible user name?

As I understand it, a page like www.facebook.com/username, is not an actual file stored on disk but is shorthand for a query like: select username from users and display the information on the page. How does Google know about every user, this gets even more complicated when things like tweets are involved.