Twitter search is one of the most powerful tools for TV programme makers because many users are in the habit of discussing the shows 'live'. These streams can be used to assess buzz, find out what people think of the content, and even to find out what they might want to happen next. They can also be used to fuel buzz, which has been used particularly well by ITV to promote Downton Abbey and the X-Factor.
This kind of information can be used to create new storylines, engage in conversation with viewers, offer options for different plotlines and so on. It can also form the basis for spin-off shows, many of them internet-only with incredibly loyal fanbases.
Another important use of social media for TV and filmmakers has been crowdsourcing. This isn't just about creating new plotlines, it is also about finding resources such as actors, cameramen, and even funding.
There is a steadily increasing number of internet-broadcast 'TV shows' made on a shoestring. By using social media, they have benefited from being independent of big production houses and PR people. Chat show hosts in particular benefit from guests who have been freed from their PRs and can give a more candid interview than they would on traditional television.
People will always watch TV for entertainment, but it is set to become a much less passive medium in the future. As Philip said, with social media, anyone can make a show these days. It's great to live in an age where we get to find out what they make next!