You think all his users are using his app because he's young? I think that is unrealistic; they use it because it provides value. Yahoo! just wants to incorporate that value into their own offerings. How is this surprising? To make something great, you have to bring lots of good things together.
No, I think they started using it because they heard of it (because he is young) and then stuck with it because it provides value. Whether that value is strong enough to essentially start again with a whole new app is debatable.
