That sounds like registered users, which may be completely different than active users. The number of registered users tells you very little without also knowing the number of active and/or concurrent users and what type of activity they're doing.

I agree. Look at engagement rates and how the curve looks on that end. It is much more important than stale users who registered but never used your service/product.

Either way, I'd say a few million users is pretty impressive.

Agree that total users shouldn't be looked at without % of actives, but he's not asking if it's impressive from the POV of evaluating the business. He's asking in terms of evaluating an engineer who handled the scaling from a technical POV.

Yes, but depends on what kind of system. Our company has written telecommunication systems to handle over 50M users and yes we do brag :)

No offense meant to you personally here, but only in Silly Valley could someone seriously ask such an inane question.

Over four years, plenty of user bases go from the millions to zeroes. Even maintaining a user base over that time should be considered an impressive feat.