Earlier quoted context omitted.
To expand on simondlr's point, Twitter will optimize the site for maximum exposure of ads, not user's status updates. The highest paying ads will surface to the top over the highest quality status updates.
Sorry but that's crazy. Twitter is NOT going to compromise the integrity of the entire site for ads. Google haven't. Facebook haven't. Microsoft haven't. And thousands of other sites haven't.
As for Twitter... they've done better than most, so far. But at some point, they'll have more pressure to show revenue. And then... how do they make money, again?