For example, I load tested my last rails app (load testing is my background-I have consulted for major pharma, tax, and other companies) and on a wimpy inherited server, processor maxed out first for the rails processes. I popped in a second CPU, then was able to get to the point where we could support 200 concurrent users, which was all we needed at the time.
I have seen limitations on bandwidth, firewall problems, load balancer misconfiguration, poorly written queries, memory usage,processor usage, thread deadlocks, and licensing limits (max database connections on a trial license) all be bottlenecks. At the larger scale, yes, database is more often the problem, but it's not clear twitter should be using a database in the way they are, anyway. Your statement goes too far.