I hope this comment is a troll and you don't actually believe that you could run Twitter on a single server. If you are interested in why this is true I suggest you get a job at a company that has to serve consumer scale web traffic.
The fact that you think "consumer scale web traffic" is some magical thing is exactly what I am talking about. Have you ever heard of TPC? They have done benchmarks of database driven systems for a long time. TPC-C measures performance of a particular write query, while maintaining the set ratio of other active queries. The top non-clustered result right now does 142,000 new orders per second. Yes, a single server can handle 300k reads and 6k writes per second.
I appreciate that you believe what you are saying, but TPC-C doesn't measure anything at all relevant. Having worked both on the enterprise side building WebLogic and on the consumer side at Yahoo and Twitter I can tell you definitively that you are wrong about the applicability of that benchmark to serving large scale web applications. The database and server you are talking about could not do 300k timeline joins per second, or 300k graph intersections per second or virtually any of the actual queries that Twitter needs to operate. All reads are not created equal. Checking the latency profile, it is horrendous even for these simple transactions — hundreds of milliseconds! Worse, there is no where to go as the service grows.
It would be great if you were right and all the big web companies are wrong. I can assure you that it isn't the case.