I think what you have said is true (to sum extent, i think there are 3 demographics though not to)as i have seen people not within those 2 demographics you mentioned have no interest to use twitter, however those 2 demographics are very large. There are alot of people who are students and alot of businesses and people looking at connecting for monetary purposes.

The thing is, when people who do not fit into those demographics sees that it is possible to make money and easily communicate,it drives interest, attention and people not longer within those 2 demographics to try and give it ago. Some may be successful in achieving what they want some may not.

As for the third demographic, lots of celebrities use twitter and i know people who actually just signup to follow and get the latest (semi truthful news as they put it) through twitter. They have a client on their fone and when a twitter comes from kevin rudd, or oprah or whoever they check it, give it a read and get on with their lives never writing a tweet .. ever !

I wasnt a big fan before but after using it, i find it totally indispensable in connecting and finding out what is going on. Hell i clicked on this link through a twitter tweet from @newsycombinator :P