My thoughts as well, I love this!

Easy to do, easy to implement but hard to bypass. Also it tells me something about the network that is not vying for a slice of the attention economy and isn't going to do everything it can to keep me on the site.

Don't underestimate the stubbornness of "get rich easy" people when it comes down to cheating etc. Even if it's not easy or cost effective, if this was going to be actually viral, they would tap real phones in click-farms to game the system. And do it once a year.