Nielsen is now measuring what it calls the “unique audience” for Twitter posts about television, providing a more complete view of the phenomenon known as social TV.
Twitter drives TV viewership. However, the cable-less such as myself sometimes just watch the tweets. For example, Buzzfeed Sports does a great job in tweeting out game highlights in GIFs. That’s basically like watching a real-time version of Sports Center.
While Twitter drives eyeballs, I think in many cases it also keeps people with Twitter itself.