Gathering what people are saying is relatively easy. Figuring out the meaning accurately is extremely difficult, and as you add longer text the difficulty grows.
Something like Siri wasn't developed in a void, the technology behind it took quite a long time, with many millions of dollars, and that is only aimed at "understanding" short sentences.
What you're asking for might cost a pretty penny to develop, it isn't just a "one guy in a basement for a long weekend" type of concept, it is a legitimately hard Computer Science problem with no easy outs.
If you were able to scan social media and extract WHAT people are talking about then it is definitely a monetizable idea. However from zero through to profit is a very long ride.
Maybe contact the NSA and try to negotiate re-using their in-house technology?
