Twitter is rolling out its "Nielsen TV Twitter Rating" system in the US today. The metric was first announced in 2012, and was developed in partnership with TV ratings-tracker Nielsen. It will show both the number of tweets mentioning specific shows, and the number of accounts those tweets reach.

That second statistic is of particular importance to Nielsen, Twitter, and advertisers. The New York Times calls actual tweets about a show "the tip of the iceberg," and quotes Andrew Somosi — chief executive of SocialGuide, acquired by Nielsen in late 2012 — saying "the full iceberg is the extent to which people are seeing those tweets." The New York Times uses the example of Breaking Bad's finale. The show was mentioned in 1.2 million tweets but those tweets reached 9.3 million Twitter accounts.

SocialGuide's vice president for product, Sean Casey, told the NYT that his company has "a variety of techniques" to record tweets about specific shows, and that results would be carefully filtered. Casey's claims will mean that, for example, references to NBC's show Community are picked out, while mentions of the societal construct "community" are discarded. In the future, Casey says, the rating system will be able to pick apart data further by working out the difference between a celebrity's tweets about their own shows and tweets from the viewing public.

Nielsen and Twitter aren't saying how many TV networks or advertisers are currently signed up to use the new rating system, but its introduction shows the micro-blogging platform continuing to cozy up to TV as it courts investors. As The New York Times notes, Twitter's IPO mentioned television 42 times.