First - If you want to "write software that will actually make a difference", your best bet is to stop worrying about advertising markets and twitter clones and

First - If you want to "write software that will actually make a difference", your best bet is to stop worrying about advertising markets and twitter clones and focus on exactly that - finding some problem in the world and fixing it. Second - the advertising dollars in the valley are not zero-sum, because the potential market, while finite, is not nearly what it could be. How to do effective brand advertising or video advertising on the web are very much unsolved problems, and whoever solves them will substantially increase the size of the pie that everyone is fighting over. These aren't technical problems, but they could have technical solutions (Ooyala in the video advertising space is a good example).