This is an argument I've tried to put across with my site http://twittrading.com which is a pretend live stock market based on sentiment analysis of tweets.

If you could buy/sell Beiber or Gaga this system might actually work. Anything else and you've not really got enough to determine individual stocks.

If your going on the sentiment of ALL of twitter as a driver for buy/sells then maybe that might work, or maybe it will just be a useless :)