The notion that social media buzz can drive television ratings or ticket sales is not a new one. As early as 2006, the producers of Snakes on a Plane ordered several days of additional film shooting after Internet chatter thrust the low-budget B movie into the national conversation. (When it was finally released, New Line Cinemas considered the box office numbers a “letdown”.)

In February of this year, Brian Stelter wrote a New York Times piece suggesting a direct correlation between social media conversation and record TV ratings for major live events. He interviewed several television execs who suggested that the Internet lends a “watercooler effect” to TV watching, allowing viewers to discuss the live events with their friends while simultaneously creating organic buzz around the event, thereby driving new viewers who might not otherwise have tuned in.

But though industry watchers were able to offer up anecdotal examples of increased engagement and viewership, there were no studies that drew direct correlations between social media discussion and television ratings.

Last week, however, Nielsen published findings that not only found a correlation between the two, but also assigned a number to it. “A few weeks prior to a show’s premiere, a nine percent increase in buzz volume correlates to a one percent increase in ratings among [18 – 34-year-olds],” a Nielsen blog post put it. “As the middle of the season approaches and then the finale, the correlation is slightly weaker, but still significant, with a 14 percent increase in buzz corresponding to a one percent increase in ratings.” Social media had its highest effect with women and a significantly lower engagement for men over 50.

Though the findings may give television networks an incentive to direct additional resources toward social media platforms, they certainly weren’t resting on their laurels prior to the Nielsen report. Two weeks ago I was sitting on a long bus ride between New York City and DC when I noticed several people I follow on Twitter checking into the PBS documentary Prohibition on GetGlue. GetGlue is a social networking site that allows users to “check in” to media they’re consuming in real time. The site provides a platform for viewers to discuss the shows they’re watching, giving them points and stickers as rewards. Users are often encouraged to share their check-ins and viewing habits by posting to their social streams on both Facebook and Twitter. According to a blog post the company published last month, GetGlue saw 11.7 million check-ins from its users during August alone. And it “now has over 1.5M users and has a database of over 200M ratings, reviews, and check-ins.”

Prohibition, the much-anticipated Ken Burns documentary about the 13-year ban on the sale and manufacturing of alcohol, has garnered over 28,000 check-ins and more than 4,600 “Likes” to date. On the night of my bus ride, it ranked among the top ten shows, with more check-ins than The Simpsons. Kevin Dando, PBS’ director of digital marketing and communications, told me in a phone interview that those numbers are a reflection of PBS’ heavy involvement with GetGlue and other social media platforms. “There are several different stickers that people can get with Prohibition,” Dando said. “You get one for checking in and watching a preview, you get ones for each of the three episodes, and then there’s one for people who have watched all three nights.”

While PBS was certainly trying to tap into GetGlue’s network of entertainment-adoring users, it’s evident that the engagement GetGlue can drive on Facebook and Twitter is what provides the main incentive for using the service. GetGlue’s internal metrics, Dando said, estimated that the check-ins were seen by over 8.5 million people, mostly as a result of people publishing their check-ins to their Twitter and Facebook feeds. (The estimates, in part, come from Twitter’s API.) And once a check-in has been broadcast to someone’s social stream, it often causes further conversation within that person’s network. Because each check-in included a hashtag, Dando was able to watch those conversations blossom in real time.

“I’m seeing people respond to those check-ins with things like, ‘Oh yes, I forgot that was on tonight,’ or ‘I’m DVRing either Prohibition or the baseball game.’ So it was another reminder that the show was on. In some cases people were responding to the tweets with ‘I didn’t know…’ followed by a fact from the show itself.”

For PBS, a large incentive to make those pushes is to reach new audiences that may not already be watching PBS. When a regular PBS viewer begins sharing his real-time viewing habits to his social stream, he (potentially) alerts other (potential) viewers who wouldn’t have otherwise been aware of the program.

This may have been the case with Prohibition. With the obvious caveat that it’s nearly impossible to draw a direct correlation between social media engagement and viewership, the first night’s installment of the series averaged a 2.6 household rating, which was well above the 1.2 household rating Burns’ last documentary, The Tenth Inning, saw on its first night. According to a PBS press release, this “was 189 percent above the PBS overnights’ primetime average for the 2010-2011 season.”

I interviewed Dando before the release of the Nielsen report, but as the person who runs PBS’ over-900,000-fan-strong Facebook page, he’d already seen a fair share of evidence that social media drives viewership. “People are using our Facebook page in an interesting way by subscribing to our feed for reminders about our television programs and the availability of those programs to be streamed online,” he explained. “As soon as something is put up on our Facebook feed, I can see within seconds that many people are going to our online streams” to announce they’re tuning in.

PBS’ local affiliate stations have noticed the effect, as well. Dando told me that he is regularly contacted by the affiliates before their programs air, each one asking him to plug the show. Because Facebook allows geo-targeting with its Wall posts, he’s able to promote local shows without cluttering the feeds of people who live outside of their broadcasting reach. He’ll send out 20 to 25 geo-targeted reminders in any given week.

But one of the largest indicators of social media’s effect on viewership can be found in surveys PBS conducted with its Facebook fans, asking them what kind of content they’d like to see published to the PBS Facebook page. The most requested piece of content?

Tune-in reminders.