This is not necessarily true for social apps where number of active users are actually a feature. Your app itself might be great, but if you don't have active users, say good bye to ever growing big. This is also true on the contrary. If you happen to have started some service before others (Craigslist, dare I say Hacker News), even though the app itself might not be as good, because of the people you have on the site, it will win out against superior apps that have smaller number of people.