I don't think it's the number of users necessarily, it's about the expectations. People seem to expect Google quality search behind every search box on the web. The search needs to find the relevant information, first time, every time.

So it's more about being able to find what people are looking for, not the number of users you have. And to be fair, almost all search solution currently available will scale beyond what most of us need in terms for number of users or the size of our datasets.