The disconnect here is that only a tiny percentage of sites on the Internet have the requirements of Twitter and Facebook. They are outliers. Using them as an example here is ridiculous.
You don't need to have Twitter's userbase to have Twitter's data problem. Look at someone like FlightCaster -- they're using a NoSQL database to handle a huge amount of data to be useful to even their first user.
With the advent of cheap, reliable, available commodity hardware and network access, previously difficult data problems are solvable. SQL doesn't make sense for all of those problems.
