It seems to me that, say, three years ago building an absolutely kickass mobile client, or just iPhone client, should have been a major priority (versus, say, randomly thrashing the timeline interface over and over).
Hey don't knock timeline! It's incredibly valuable for discovering that the two most important times in most people's lives are the year they were born and the year they joined Facebook.
Advertising is largely the art of throwing shit at a wall and seeing what sticks. Yeah, with a lot of tools and the information Facebook has about their users, we can perfect the art of how to make shit stick.
This is the classic failure of big media: treating your users like an audience (eyeballs). Facebook understands its users interactions, designs around them and is doing brilliantly at keeping them engaged, then they bring in people from the advertising world and treat them like an audience.
They should be monetizing the edges, not the nodes.