The shiny new toy syndrome lasts a month or two at best these days and then the hot new app crowd moves on. In the wake of that you will have built a base of loyal users who will be a fraction of your total users. 30% is a good number. If you kept more than that, you are doing great. If you kept 10% or less, you’ve got serious problems.
USV invested in Twitter, Tumblr, and Zynga the same summer (2007). Zynga and Twitter took off fast. Tumblr was a slow build. They are all Juggernauts now.