LA Times: For its latest report, Barracuda Networks set out to determine how many people with Twitter accounts actually use the service. For the purposes of the report, it defined a "True Twitter User" as someone who has at least 10 followers, follows 10 people, and has updated their status with at least 10 tweets. Only 21 percent of Twitter users met all three criteria.
In addition, 17 percent of Twitter users have no followers, and 74 percent have less than 10 followers.