For several reasons you may annoyed when you have follow someone on twitter, but they don't follow back you. Sometimes I also feel the same. This morning I just arrived to my campus, then go to the net by login on twitter and searching something on google. I found the interest site that we can use it to knowing who are don't follow back us, and we can unfollow them easily if we want. I also use this nice tool to unfollow who don't follow back me. Yeah just because I annoyed to them 
. Here the site
. Here the site