> Scale and Money

This is too simple of an answer. By these metrics Google+ would be a success. Also, the total funding amound of DDG is 13M (see Crunchbase). While that is serious money, that's not series D money.

Moreover, there are counterexamples of companies that did build an app in a relatively short amount of time, kept a small team and made millions per employee. I don't know of many [1], but I think WuFoo is an interesting example, founded in 2006 while SurveyMonkey was founded in 1999. WuFoo exited for 35 million in 2011 with a team of 9, I think (there's a YC YouTube video about this somewhere).

Disclaimer: I'm an armchair quarterback.

[1] I don't know of many tech companies in general.