Yeah it's tough for me to keep track of which apps are/aren't significant because I'm not in the loop with every community out there.

I'm guessing the users are distributed among small pockets in a network of people who all know each other -- like how in the 1990s there were pockets of "MSN Messenger" and pockets of "Yahoo Instant Messenger" and pockets of "AOL Instant Messenger" users randomly scattered around various schools. OR like how Orkut grew so precipitously in Brazil.

Or it's a generational thing -- I think there is a certain age group of people in the US who used Yik Yak, another who uses tbh and co, and so on.

I would generally trust Facebook employees to tell me which apps are "real" / growing since they have Onavo surveillance and other sources (https://www.wsj.com/articles/facebooks-onavo-gives-social-me...). But aside from those data, I'm not sure how else to keep track of which apps are "real" / big.