Twitter Metrics To Measure For Business

You spend hours on Twitter, for your business.  How do you measure your ROE (Return on Engagement)?  Measuring the benefits of investing in social media marketing is much more difficult than with other traditional channels.  However, an easy start includes monitoring the metrics below over time:
  # of followers
  # of tweets about brand
  # of @ replies
  # of retweets
  Click through rate
  Sentiment