Source: Flickr/Maurizio Pesce.

Facebook (NASDAQ:FB) and Twitter (NYSE:TWTR) are the go-to platforms for discussing live TV. And more than anything else on TV, people are discussing sports on social media.

Actually, that's more than everything else on TV, according to Sean Casey, senior vice president of product at Nielsen Social. "Sports events comprise somewhere between 2 and 3 percent of TV programming in any given month but generate close to 50 percent of the Twitter activity," Casey said in an interview with The New York Times.

That trend is not confined to Twitter. During the World Cup, Facebook saw nearly twice as many posts and comments as Twitter about the global sports event.

Snapchat may be the biggest threat to Facebook and Twitter in the growing second-screen market. The company's latest hire, Eric Toda, is a former director of digital at Nike, and he was brought on to help with Snapchat's partnership team. That could mean more partnerships with sports teams and leagues for the photo and video sharing app. 

Snapchat may have started as a small app that lets users send self-destructing photos and drawings to one another, but it has grown well beyond that. Now its 100 million monthly active users (up from 30 million last November) are able to send video, create and share a video/photo diary of their last 24 hours, and use it as a regular text messenger. It's simple interface and private sharing capabilities have resonated with a younger audience, which makes up the majority of its user base. Now, it could leverage its video-sharing platform to offer an excellent second-screen experience. 

Not just your story; Our Story
This summer, Snapchat rolled out Our Story, a new feature that provides a collaborative live stream of big events like the World Cup Final or Electric Daisy Carnival, a music festival. The goal of Our Story is to connect moments of a shared experience, which is exactly what tweeting and posting on Facebook aim to accomplish during television.

Snapchat's first Our Story was "EDC Live." Source: Snapchat's YouTube channel.

Fans are able to post pictures of themselves or their surroundings as they prepare for or experience the big (or not-so-big) event -- which is already popular on social media. (Remember Warren Buffett's post on Twitter before the Breaking Bad finale?) Snapchat has a more streamlined user interface than either Facebook or Twitter, though, which makes it ideal for sharing in the moment. All users have to do is launch the app and start recording or snapping photos. 

But if Snapchat can partner with sports leagues, teams, or television producers, it can add exclusive video and photos to Our Stories. You could be at a football game, for example, and Snapchat could push you an instant replay of that last pass to Julio Jones. Or you could be on your way to the baseball game, and you could get exclusive footage of the 500-foot blast Mike Trout just hit in batting practice (because you're obviously stuck in L.A. traffic).

This kind of partnership is not unprecedented. The NFL has partnered with Twitter since last season to bring customized content to the platform. Twitter added curated timelines for every NFL game this year and is pushing heavily into sports.

But Snapchat seems like a more natural platform for the second screen, and sports in general. Snapchat can use geofencing to collect and display snaps from stadiums or red carpets and provide a seamless stream of content for viewers stuck at home. Videos auto-play, and there's no need to click a picture to see the whole thing. Contributing is as easy as sending a snap, and reactions, like sports, are ephemeral instead of archived.

Big TV money
There's no doubt that engaging television audiences has a lot of potential in attracting some of the television ad dollars to Facebook or Twitter. If Snapchat is able to make a better second-screen experience, however, those ad dollars might flow more toward the app than to the big social platforms.

eMarketer expects the absolute growth in television ad spending to continue to outpace digital video ad spending for the next five years, but digital video spending is expected to triple in that time. Facebook and Twitter have both been pushing heavily into video and video ads recently, and Facebook saw an average of 100 million new video uploads every month over the past three months. That's a good base to attract the growing number of video advertisers.

Snapchat, comparatively, has approximately 67 million users sending and receiving videos and images on a daily basis, so it probably tops Facebook's video uploads. Of course, most of those videos aren't easily monetized -- Facebook has the same problem. But premium video footage -- say, from a sports team -- could be monetized by placing an ad in the middle of Our Story.

That's ad money that would not going to Facebook or Twitter. But what's worse is if Snapchat cuts into Facebook and Twitter's audience during live events -- a distinct possibility if Snapchat and Toda can deliver.

Adam Levy has no position in any stocks mentioned. The Motley Fool recommends and owns shares of Facebook, Nike, and Twitter. Try any of our Foolish newsletter services free for 30 days. We Fools don't all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.