Angry Birds lends itself to a product, they have a very specific set of characters and goals/tasks, as well as a back story. DrawSomething is much more broad, there is no direction in it; the only goal is to essentially play pictionary; I can do that at home. Where they've changed the game is that you can now do it easily with your friends hundreds of miles away. That simplicity is what limits it, but also makes it so popular. You can draw a parallel to something like pinterest; is it a hugely popular site, absolutely, but what can they do now to make money? license it? develop a product? sell you products that people pin? so far the answer is no for all 3 of these options (the last option is something they're working furiously on to monetize the site, but the volume and randomness of items that people pin make it so fragmented that it's difficult...)

By the way, it looks like Zynga is in talks to buy OMGPOP, looks like one big hit is all it takes!