A think tank is usually comprised of a group of people hand selected to solve a particular problem or to do research on a problem. We don’t usually open up the think tank to just anyone.
Crowdsourcing opens up a question or inquiry or research to everyone, or perhaps to a select special interest group, those who can offer authoritative data. People are drawn to contribute knowledge – whether it be of the pothole status in a given neighborhood, crime rates, weather patterns, or crisis intervention. This knowledge has very often not had a home in the past because there was nowhere to put it, or it might have to be vetted first (made into authoritative geodata) before being committed to the total database of knowledge on the given subject.