Teenagers represent 9.5% of the US population meaning that 3.8% of the US population (using their 40% figure) have "used" their app. Alternatively the 240 million registered users would imply about 3.5% of the total global population has used it.
I can say that I honestly have ZERO doubt that "kik" would never be ruled famous by any possible measure.
To be even reasonably certain in this case, I would need case law of similarly well-known brands being challenged. Do you have some? Capturing 40% of their target market seems pretty well known to me, but again, that's meaningless without the case law.