I would argue that you need monitoring significantly sooner than 500,000 users. I guess, until then, you just use Twitter noise for monitoring? Seems like pretty bad customer experience.

If I have something in an environment that I would start to consider "production" (i.e. someone relies on my product to do something regularly), then I'd have monitoring regardless of the number of users. Even something as simple as, "Am I returning valid data from GET /"?