I don't know. Scaling as effectively as Twitter doesn't seem like a particularly high bar. They spent a year or so after they got big being down almost as much as they were up. The "fail whale" was just about the most common thing to see on twitter.com.
Of course, now that Twitter is established, you cannot repeat their mistakes if you want to succeed.
Is there a single example of a service which failed to scale? Once you have user growth, you can get investors to fund scaling regardless of the technology choices made the beginning.
