Trying to profit on the backs of regular users won't work for Twitter. Charging a regular fee, loading up on enough ads to keep investors happy, or anything else like that is a quick way to lose a bunch of users. Instead, Twitter need to think like Google or any other ad network. They should charge those who make money by tweeting. The industrial tweeters, the people flogging their sites or albums or hair products or whatever. If you've ever hired a social media consultant, Twitter should be charging you thousands of dollars a year. I'm sure there's some level of "tweets read per week" that would catch Kim K while sparing most high school tech bloggers.