Twitter and Nielsen, the TV ratings tracker, rolled out Nielsen Twitter TV Ratings today, a service that tracks Twitter activity about specific shows.

But it's not just tweets mentioning a show that are counted; the reach of those tweets, i.e. how many Twitter users view them, is also tracked.

Andrew Somosi, CEO of Nielsen-owned analytics company SocialGuide, told the New York Times that that data is what's really important - "the full iceberg," where the raw numbers of tweets are just the tip.

Take a recent episode of Grey's Anatomy - there were just 225,000 tweets about the show but they were seen by 2.8 million other Twitter accounts.

The next big thing

Twitter is touting television in a big way. As the Times pointed out, the IPO prospectus the social network published last week mentioned TV 42 times.

The Nielsen Twitter TV Ratings can help media companies track Twitter engagement relating to content, and help validate Twitter's "social TV" movement at the same time.

The ratings might prove quite useful. In fact, they can be delivered overnight and techniques are in place to ensure casual, unrelated mentions of words like "scandal" aren't counted erroneously as relating to the TV show of the same name.

They also plan to add the ability to distinguish an actor's own tweets about a show, making it possible to suss how influential those posts are.

The hard questions

But it's unknown how many companies have shown interest in the new ratings, and Twitter and Nielsen aren't saying.

It's also basically impossible to tell if there's a direct correlation between Twitter chatter and viewership.

For example, does lots of tweeting about a show result in more people watching the encore broadcast the following day?

The data being provided in the Nielsen Twitter TV Ratings can't answer that question, but it might be a good start nonetheless.