I've seen this question(s) just now on Joshua Porter's blog, and not knowing where to put it (too long for Twitter, not possible to call this part out on FB), I figured I may just as well post it here. The whole deal with Twitter is "ambient awareness" so what he asks above is a pretty relevant stuff. If you didn't know how many people "heard" what you have to say, would you stop talking? Probably yes, after a while. Not knowing who/how many/what your audience is takes away a critical feedback loop (even if they don't say anything) that let us know that we are part of the social group.
More interesting to me is to think about the quality of statements vs. confidence levels (of which they are delivered) if a person knows that he/she has 3,000 followers. As the saying goes, nothing succeeds like a success (a common translation of the power law), and the person who already has a giant number of followers will have more, which will make him/her think/feel that what they are saying is important, giving them confidence, and making them say more freely and authoritatively more stuff. On the other hand, if someone has 50 followers, they know that not a lot of people will hear what they are saying, which makes them less motivated to broadcast their opinions, and also makes them less confident about the validity of their opinions. So I kinda think that quality/validity of an individual's opinions is a consequence, and not the cause of the large followings. Because, who else is going to validate our opinion if not other people?
Anyway, Josh's post is very good, read it here.