Are you sure a court wouldn't consider that qualifying as a famous mark? I'm not going to dig into case law, but my guess is it very well could be.
"Evidence relevant to the fame of a trademark may include sales, advertising and revenue figures; geographical scope of use; channels of trade; registrations in home and other countries; past enforcement efforts; and the results of consumer recognition surveys (provided the survey methods are approved by the courts in that jurisdiction)." http://www.inta.org/TrademarkBasics/FactSheets/Pages/FamousW...
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.