I run a much smaller app that's bootstrapped, but we have hit top 100 free apps in the App Store before, and have been featured in the section our app is in, so I'd like to think I've learned at least a couple things -- tweens and teens more or less dominate app store explosive growth with regards to social apps.
I think the sticker chat is definitely something that those teens are into. That being said, those same teens mostly just care about followers and likes, which is why Instagram is quite the 100 pound gorilla in the room. Believe it or not, there are tons of folks on IG with tens of thousands of followers, who on twitter or Facebook have 10-100 followers. It is the de-facto mobile social network.
Honestly Path is a damn beautiful app, but because Path does not have enough content, people will leave. Period. It would be interesting to hear what the engagement numbers are on that 6M.