Tunerfish, a new social TV service developed by employees of Plaxo, the address book/social networking service that Comcast acquired back in 2008 (note: Tunerfish is a business unit of Comcast Interactive Media--for more on the service, see the articles published on itvt.com, May 24th and June 28th), said Monday that it has been tapped by NBC News to power a promotion that rewards viewers of the news magazine show, "Dateline."