Sentiment analysis was a big topic and is still being used. But results are very specific. Most investors won't tweet while they're losing money so the short term information gain is small.

Where it can help is to determine the state of the economy. Same as with image recognition for satellite pictures (traffic patterns, footfall at stores), those indicators can give you a faster idea if something's going wrong with the economy than classical indicators. But I have yet to see an accurate one.

Any links with more info would be appriciated :)