Akamai handles 15-30% of all web traffic on any given day with 200k-300k servers...does Facebook have that kind of load in a day?
The problem is mostly a CDN one, crunching data and writing to the servers are second to actually serving users fast.
For example, I don't necessarily need to see the latest posts, comments, etc. so you can queue up the writing and sync data later, but when I hit fb.com, I have to see stuff...