First off, I don't give 2 shits about twitter, so I don't care if the numbers are skewd in either direction. This is more of an interest in seeing how SV stats/metrics are just a game. Just so that's out there.

A lurker isn't an active user in my opinion. Maybe that's not the same understanding as accepted definition. The lurkers might be absorbing some of the ad content, but they are not helping create new avenues for ads to be shared. Twitter's ad share surface area would increase tremendously if every user was actively producing tweets. That's the only metric that they are concerned. They don't care about how many people actually see the ads once they are there. They make their money on the potenial eyeballs alone. Lurkers are not helping increase those numbers.