(Read More: Facebook vs. Twitter, Who Has Edge in Mobile Ad Wars?)
While there has been a lot of talk about how chatter on Twitter is impacting live TV viewing, and several companies have tried to quantify it, until now the value of all that tweeting has been unclear.
Nielsen said the rating will give "TV networks and advertisers the real-time metrics required to understand TV audience social activity." However, the company hasn't revealed exactly what it will quantify, or what it'll look like.
Bottom line: This is a good thing for Nielsen, which has long been criticized for an archaic measurement system that overlooks key places people watch—hotels, bars, gyms etc.
(Read More: 13 Twitter Tweaks We'd Like to See in 2013.)
By officially partnering with Twitter, Nielsen has indicated its commitment to transitioning from its set top box measurement to a fully integrated picture of how people watch TV—and what that means for advertisers. Nielsen and NM Incite recently acquired SocialGuide, which captures Twitter TV activity. By working with Twitter directly, that means even better data.
What does this mean for Twitter? Although financial terms of this deal weren't disclosed, it likely means an additional revenue stream from Nielsen. The data should surely drive more advertising on Twitter by marketers looking to reach consumers on both platforms when they're watching.
(Read More: Twitter Offered Instagram $525 Million Deal: Report.)
CBS has already been working with Nielsen and Twitter to experiment with how to use this kind of data. Both CBS and Fox were quoted in Nielsen's press release, with Fox Networks Group's CEO Peter Rice saying:"Combining the instant feedback of Twitter with Nielsen ratings will benefit us, program producers, and our advertising partners."
Why? The more information the networks can give advertisers, the better they can target ads, and the more the networks can charge.
What does this mean in practice? Advertisers will likely be able to use this data to show ads for the same product on Twitter as they are airing in the TV show. That cross-platform reinforcement makes messaging much more effective.
(Watch: Twitter's Dick Costolo on the Future of TV.)
Digging deeper, advertisers will be able to cross reference viewers—are people who tweet about their show also following certain brands or tweeting about certain products? The ability to get a full picture of their consumers on Twitter has wide-reaching implications for targeting ads both on TV and within the social platform.
—By CNBC's Julia Boorstin; Follow her on Twitter: