So I have to admit upfront that I don't have an Instagram and I'm not sure how the number correlates, but I currently have shy of 600 "friends" on Facebook made up of several different social groups (high school, college, law school, various internship programs and jobs, family, online weirdos, etc.) as I developed since like 2007. Each social group kind of began with the low hanging fruit and grew outwards as the social network matched friends in common.
On the Twitter side (which might be more similar? I don't know if Instagram is mutual friending or solo following), it kind of began that way but stalled out at 48 followers because I have no reason to use the account without followers and I have no reason to build followers because I don't use it.
Basically to get 500 people to follow your account, they need to see it and be interested in the content. The only surefire shortcut is to know 500 people who say "hey that's Appa, I'll follow them", otherwise you just have to post content for people to look at, and find a way to get people to go to look at the content, and build up followers naturally.
But is online follower count a real measure of social strata that college-age folks take seriously?