It benefits people because everyone can blog about different stuff
it doesn't
Letting teachers know how Twitter can benefit them in the class room is a good way to get more teachers on Twitter.
A lot of people have a Twitter. Four out seven people in the world have a Twitter.
To message people on Twitter, you must have an account.
It is highly likely that there are people making money on Twitter apps. For example there are many Twitter clients for the iPhone, some free, some ad supported, it is a fair gues that all of these people are making some money. It is all a matter of finding a way to interact with Twitter that provides people some benefit over the free standard interface. If you do it well enough people may pay (or tolerate ads) to use it.
it doesn't it just tells the whole world that you have to go poop
I have been stalked on twitter but most people aren't.
Yes, you can actually know if someone retweet or mention you in twitter. It is a default feature in twitter and every tweeple (twitter people) can know who followed and mentioned them.
Yes, the site Twitter is a social network
No he did not, "his" Twitter was created by his PR people.
190,000,000 people follow Twitter.
As of March 17,2011 8,202,406 people are following him on twitter