A study by researchers from HP's Social Computing Lab shows that Twitter does very well in predicting the box office revenue for movies.
[Researchers] found that using only the rate at which movies are mentioned could successfully predict future revenues. But when the sentiment of the tweet was factored in (how favorable it was toward the new movie), the prediction was even more exact.
But as someone noted in the comments:
Works fine until people realize it works, then they start gaming it, and it stops working.