If someone mentions that they were responsible for overseeing this growth, in their resume should I be impressed? i.e. they built systems which scaled to meet this growth.
That sounds like registered users, which may be completely different than <i>active</i> 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.
No offense meant to you personally here, but only in Silly Valley could someone seriously ask such an inane question.<p>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.