It’s hard not to feel a little sad for ESPN. Not too sad, of course, given that it is still one of the most profitable TV networks on the planet. But when you are on top, the only place left for you to go is down, and Facebook just debuted something that is aimed straight at the heart of ESPN’s biggest weakness—namely its lack of a real-time digital strategy.

Facebook’s new feature is called Sports Stadium, and it’s essentially a real-time hub for sports-related commentary and game scores. In a lot of ways, it’s like the sports version of Twitter except on Facebook, and that means ESPN isn’t the only one with a target on its back. It’s also a shot across the bow of both Twitter TWTR and Snapchat—the latter’s Snapchat Stories are popular for big sports events.

As the giant social network points out, it has 650 million sports fans. Even if not all of them care deeply about commenting on sporting events, that’s still a huge pool. It’s five times as many people as watched the Super Bowl last year, and twice as many as Twitter has on its entire platform. ESPN has fewer than 100 million subscribers (and shrinking).

In much the same way Twitter has with its various sports-related offerings—including deals with the National Football League and the recent launch of its Moments feature—Facebook is trying to become a second screen for sports, the place where people go to “talk trash” about the opposing team, or watch video highlights, or catch up on what the score is in their favorite match.

Facebook Sports Stadium's tabs for live matchup and friends' posts.

Sports Stadium looks and sounds a lot like an app, but one that functions inside of the social networking platform instead of being a standalone (although you can definitely see how it could become a standalone app if Facebook wanted to do that). It has tabs so users can switch between commentary from their friends, commentary from experts and teams, and live sports scores, as well as live play-by-play coverage.

What does that sound like? It sounds a lot like what Twitter turns into whenever a sporting event is on, but it also sounds a lot like ESPN. Expert commentary, posts from the teams, contributions from viewers, and live scores and play-by-play. Except it’s on Facebook, and it’s free.

How Fox plans to take on ESPN

Obviously, ESPN isn’t going to crumble into dust just because Facebook introduces a new sports feature. It still has exclusive deals with sports leagues for the rights to events that extend over the next decade or more (deals for which it has spent heavily). But just as Twitter has probably eaten into the edges of ESPN’s market, by appealing to those who don’t want to have to pay for cable just to watch sports, Facebook is going to chip away at that dominant market position as well, bit by bit.

Facebook says it is premiering the new sports feature to coincide with the National Football League’s playoff games, but will be adding more sports. According to the social network, more than 350 million people talked about soccer’s World Cup during that event last year.Facebook Sports Stadium

At the moment, the ability to watch live sports seems to be one of the few things that keeps large numbers of people subscribing to cable at all, and ESPN—and its owner Disney DIS —have counted on the fact that this relationship is rock solid. But is it really?

According to a recent survey commissioned by BTIG Research, 56% of viewers said that they would be willing to get rid of ESPN altogether if they could save $8 monthly on their cable bill. Maybe for some of that group, watching live scores and a play-by-play and commentary on Facebook would be enough, combined with GIFs of the key plays they can find on Twitter.

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ESPN has already been shedding subscribers at a fairly substantial rate for some time. Based on a recent filing by Disney, the sports network has lost about 7 million subscribers in the past two years alone. Given the amount of money that ESPN gets per subscriber—the highest in cable land at an estimated $6.41 each per month—that means the network has given up half a billion dollars in revenue. And during the same period, its content costs have skyrocketed.

That helps explain why the network shut down Grantland, the high-profile journalistic effort run by Bill Simmons, and also why it laid off 300 people last fall. But while ESPN is focused on cutting costs and hunkering down, Facebook and Twitter and Snapchat are busy carving off chunks of its younger, more digital-friendly customer base.