There are several applications out there that allow you to see who has unfollowed your Twitter account recently thus giving you the opportunity to unfollow them and get back to the real business of networking. To be honest, I find this distasteful to say the least.
When I follow someone on Twitter I make sure to look at their recent tweets and interests (and their website/blog if they have one). I like to be sure that the person I'm following is interesting to me and that I may even learn something from them. If someone has followed me then I'm pleased by that fact but I won't automatically follow them back just out of obligation.
There's no denying that Twitter is an excellent marketing and networking tool for writers. It allows people to come into contact with people they would otherwise have no chance of communicating with. It helps them develop the craft of writing and spread the word (in moderation) about their latest projects. It gives people the opportunity to recommend other tweeters that their followers might like and retweet stories that may be interesting to their followers. Twitter is very much a network of give and take.
Perhaps that's the very reason unfollow tools have become so popular. If someone isn't giving you anything (reading your tweets) then why should you give them anything (reading their tweets)?
Unfortunately, for me, it's not that clear-cut. If someone unfollows me I don't take it as a personal affront or a reason to get rid of them. I may be a little miffed that they didn't enjoy reading my tweets but that doesn't negate the reason I followed them in the first place and the enjoyment I get out of their tweets.
At the end of the day, that's what Twitter means to me.