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.
http://www.tpc.org/tpcc/results/tpcc_perf_results.asp?result...
Yes.
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.
Then they ran out of horsepower on that one server. Let me tell you, the stories are pretty horrible.