There is an interesting twitter tool called twitter.grader.com in the web telling you how your twitter grade is. See on which position you are, compared with all the other twitterers in the whole world.
You even can divide the view by location and have a prying look, if you made it beeing under the top 50 twitterers in your city, region or country.
The rank is calculated by a lots of criteria such like how many followers you have and which power they have. Also your activity is defining a part of your ranking and quality.
This is also nice to find the best of the best and to get new interesting people of the community to follow.