it's like BBM cross platforms. BBM is very popular in with college students. Lot's people use it to hook up with each other. If you search for Kik on twitter it's mostly urban youth that uses it. That's why it spreads so fast. Kik is brilliant because it identified what made BBM popular and made it possible to other platform users.
What's even more impressive it that an year ago their product was completely different, and they rebranded and they won. This was what they were doing in 2009
http://velocity.uwaterloo.ca/alumni/kik-at-blackberry-devconI guess that's what they call relentlessly resourceful on HN