The consumer's move to such highly dynamic, large scale, heavily used, web applications, such as Facebook, puts incredible stress on the overall network components. From DBs, to CDNs, to app servers, web servers, and web browsers; incredibly the whole chain is holding up.
When we were trying to figure out how to open the client up to more dynamic interaction between application and user (ultimately landing on XMLHttpRequest), I had several doubts about things actually working at scale (e.g. performance, both server-side and client-side). Well... people have found a way. Bugs have been worked around, performance has been tuned, users are allowing more interesting information to be publicly available about them, and sandboxes (native or layered (e.g. FBML)) are providing true solutions that millions of users leverage all day, everyday.
When things were relatively static and non-interactive, everything working wasn't much of a surprise, but with so many moving parts now, it's actually pretty wild that it's all working.