After reading the Innovator's Dilemma, you start seeing this effect everywhere and it applies equally in people's careers. I think it's probably the same principle as the law of diminishing return - you're getting less and less out of your current path but it's still more than some other perceived endeavor.

Perhaps there is some disruptive thing Twitter could do that will eventually disrupt their current advertising model and be a true billion dollar business. It's possible they don't see it (or less likely, there is NO path for them to get where they need to be) and so they're letting it be known that they're planning on extracting increasing rents from their current income streams.