I'm on Twitter a lot. Somedays I don't tweet very often but I am always on there or have quick access to it. I currently have 145 followers and am following 244 people. I don't know how many of those followers are real people, I believe a majority of them are because I do look at them when they start following me so I can see if they are someone I would like to follow or if they are spam or even pornbots. But that isn't all I look at and there are several other factors I use to see if I should follow you or not.
1. You don't tweet about anything I care about. I don't know if you are following me because you like what I have to say or if you are just trying to acquire followers thus gain followees but I don't do that. Your tweets have to be entertaining or at least something I wouldn't mind reading. I don't follow everyone who follows me because you have to be someone I want to follow.
2. You have no original tweets. If all you do is retweet people (unless you are one of the many novelty accounts on Twitter) I probably won't follow you. I probably also won't follow you if all of your tweets are replies to tweets other people have sent you because I shouldn't have to keep up with day-long Twitter conversations.
3. You have a bad follower-to-followee ratio. You can be Neil Gaiman and have nearly two million followers and only follow 700 people. You can be Ellen DeGeneres and have 12 million followers and only follow 50,000 people. You can't be some jerkass local citizen and have 3,000 followers but only follow 60 people. You are not a celebrity and I will not participate in the stroking of your ego.
4. You tell me to follow you back. Again, I am not on Twitter to gain massive amounts of followers. I'm there to promote myself and my website while pointing out and making fun of life's little foibles.
5. You haven't filled out anything in your profile. Why do you still have an egg as your picture? Why are you even on Twitter of you are not even going to do the basic fundamentals?
I'm sure there are others but these five are all I can think of right now. There are exceptions to the rule and most other people have different rules for Twitter. Just a quick sidenote, if your website link in your profile directs me to either your private Facebook page that only friends can see or your Twitter account, you are doing it wrong.