When I first signed up for twitter, I did it to follow two people. Soon there-after, I realized I could actually use it for networking, and I started to follow more people who interested me, shared posts on writing, and who I could converse with about this whole writing thing. I don't do twitter just to get people to follow me, and as a rule, I don't typically follow people who post about things I'm not really interested in.
There are people who I've followed (using the follow-back policy) who would update every five minutes with whatever their child was doing at the moment, followed by what they would rather eat instead of writing, followed by... you get the idea. I know you can build lists, but when I log in to twitter, I want every update I'm interested in to be on my front page. I don't want to sift through pages to see what everyone is doing.
My other big policy is that if I follow someone, I do it ONLY because I want to see what they have to say. If they follow me back, great. Hopefully don't annoy them with some of the more random statements I make. But when people follow me, and then unfollow, and then refollow me just to get me to follow them back, it's kind of annoying. Don't follow me to get me to follow you.
I don't really care about how many people follow me. If you like what I have to say, great. If not, feel free to unfollow. And if I like what you have to say, I'll follow you, even if you don't follow me. :D