As someone who's spent a considerable amount of time analyzing twitter data, the most tweets come from a small cadre of "social media experts", which is the web 2.0 term for useless spammer.
Twitter isn't just for people who are famous or who acquire fame. Most of my twitter feed is from people I personally know, only some of it is from people who have thousands of followers. Twitter isn't just a broadcast medium, it's also a narrowcast medium, extremely useful for friends to keep in touch.
Seriously, #1 should have been "the bot"