I'm no intelligence expert, but I'd be surprised if the NSA would expect something like this to find a "smoking gun" proving that person X is planning Y.
More likely you already have suspects, and your suspects have acquaintances , family etc.
You would be more interested in looking at these people, who do they network with on social media, what are they interested in?
Might they be knowingly or unknowingly be providing material support?
You would probably use this information to build up a broader profile and see which avenues are worth investigation.