[ericb]

Scalability problems are usually endemic to a particular setup. There is always another bottleneck. The way you're viewing scalability is different than the reality of it.

My previous company (a web testing company) did a load test for a large tax return company. When testing started, their app could support 3 simultaneous users. When the bottlenecks were removed, one by one, the same app (framework, codebase, language and all) supported north of 10,000 users. As you test, there is usually something like a database lock problem, high cpu from a loop that needs to be optimized, bad query/data layout, memory usage maxing out, or something you are running out of, one way or the other.

The people who point out that twitter is not a standard web app and might be better served by messaging have a point also.