There are several tools to help you monitor your effectiveness on Twitter.
http://twinfluence.com/
Twinfluence calculates two types of ranks - one assigning an absolute rank (#1!) compared to all analyzed twitterers to date, and one that assigns a value and category relative to other twitterers that have more or less the same number of followers as you.
http://twitter.grader.com
These factors go into the calculation of a score: number of followers, power of followers, updates, update recency, follower/following ratio, engagement. This score is then used to compare a user against all other users that also have a score. The grade is calculated as the approximate percentage of other users that have an equal or lower grade out of 100.
http://tweetstats.com/
Detailed graphs that show tweets posted and activity.
http://www.twitgraph.com/
Simple graphs that show tweets posted and activity.
http://tffratio.com/
Shows the TFF Ratio (Twitter Follower-Friend Ratio) is the ratio of your followers to friends (or people who you follow).
Labels: measuring twitter numbers and followers, twitter statistics