All social networks suffer from 90/10 rule (90% of the content is generated by 10% of the users), so this really isn't surprising. Other posters have mentioned

All social networks suffer from 90/10 rule (90% of the content is generated by 10% of the users), so this really isn't surprising. Other posters have mentioned this as well, but I think Twitter has too much momentum. Someone trying to do the exact same thing from the end-user perspective isn't going to be able to compete.