In the neverending search to successfully monetize its massive and highly engaged user base, Twitter has gained an important ally.

Nielsen Holdings (NYSE: NLSN), the global information and measurement company of Nielsen Ratings fame, is partnering with the popular microblogging social network Twitter to create a new media metric service known as the Nielsen Twitter TV Rating. The new rating service will begin with next fall’s new television season, Nielsen said in a statement announcing the partnership on Monday.

The partnership sheds light on Nielsen’s intentions when NM Incite acquired SocialGuide, a social media analytics firm that already uses Twitter as its sole resource to develop audience engagement for TV shows, last month.

“As a media measurement leader, we recognize that Twitter is the preeminent source of real-time television engagement data,” Steve Hasker, president of Global Media Products and Advertiser Solutions at Nielsen, said in a statement.

Besides noting that the agreement is an “exclusive” and “multi-year” contract between the two companies,  Nielsen did not offer much in the way of specific details about the partnership.

For Twitter, the partnership is an important step toward establishing itself as a leader in the growing field of social media engagement with television programming.  

“Twitter has become the world’s digital water cooler, where conversations about TV happen in real time,” Chloe Sladden, Twitter’s vice president of media, said in a statement. “Nielsen is who the networks rely on to give better content to viewers and clearer results to marketers.”

“This effort reflects Nielsen’s foresight into the evolving nature of the TV-viewing experience,” Sladden added. “And we’re looking forward to collaborating with Twitter ecosystem partners on this metric to help broadcasters and advertisers create truly social TV experiences.”

Countless TV programs already encourage viewer engagement through Twitter by branding their work with hashtags and asking viewers to voice their opinions on particular characters or story arcs, but the partnership puts Twitter as the go-to social network for the entrenched, old media organizations eager to revamp their work for younger, social media-friendly audiences.

To signify the important of this trend, Nielsen’s statement included comments from executives from Fox and CBS -- two of the largest TV networks in the U.S.

“Combining the instant feedback of Twitter with Nielsen ratings will benefit us, program producers and our advertising partners,” Peter Rice, chairman and CEO of Fox Networks Group, said in Nielsen’s press release.

“The proliferation of smartphones and tablets has generated a substantial ‘connected’ TV audience that is simultaneously watching television and accessing the Internet through these devices,” David F. Poltrack, chief research officer of CBS Corporation, added. “This, in turn, will continue to create the opportunity for content providers like CBS to offer engaging interactive features for our viewers.”

Poltrack saw the deal as a hefty endorsement of second-screen viewing to complement the audience’s experience of watching a show itself, a “form of viewer engagement” which, as it “evolves into a mainstream activity,” will ultimately offer networks like CBS new opportunities to “enhance the viewing experience for our viewers and our advertisers.”

Advertisers are obviously the key here -- both for the networks and Twitter. What this new age of advertising will look like, exactly, is yet to be seen. But in the meantime, anyone with a TV and basic cable can expect a lot more hashtags in the near future.