I am a bit disappointed to have heard recently that only 5% of Twitter's Tweets are actually read. This doesn't surprise me one bit as the response rate from posts are very small unless someone has searched the key word or I have republished the Tweet on my Facebook page. I run three twitter accounts that broadcast about different topics: Jeff Cohn, DeadZones, PhotoEnforced. Through all of my accounts I have a nice organic following of about 3,000 people. However, the response rate is very low on each of the Tweets. I think this is largely because of the lack of reporting, spam and poor filter tools limit the ROI for the time spent. However, I keep publishing in the hopes that someday these issues are corrected and real time search become more of a mainstream event amongst social networks and news.
Facebook and Google are likely very knowledgeable of this fact and will soon launch features that will compete. Sharing is one of Google's biggest weaknesses and many think it can't compete with Facebook. However, I happen to believe that Android phones and potentially new service like Google Me social networking could give Google a boost on its lack of sharing features.