The two social media companies are highly competitive, even though one is currently worth $308 billion, has 1.6 billion users, and dominates digital advertising. The other, worth $12 billion, is in the middle of a difficult, messy turnaround with a part-time CEO.

Problems aside, Twitter has a couple of things Facebook doesn’t: It’s fast-moving, it’s live, and it’s public. News breaks on Twitter. Trump spars on Twitter. People watch sports and the Oscars in tandem with Twitter. As I wrote in a recent Fortune feature story about Twitter’s turnaround:

Despite Facebook’s best attempts to court so-called influencers, Twitter is still the go-to place for politicians, business leaders, athletes, and Kanye West to make important announcements. When West publicly asked Facebook CEO Mark Zuckerberg to give him $1 billion, did he make the request on Facebook? Nope—he tweeted it.

Facebook is trying to change that. The company’s “trending” tab now curates breaking news (even apparently omitting mentions of Twitter when the news breaks there).

But most notably, Facebook is pushing hard into Live video. It has prioritized live video streams in its “News Feed,” sending users so many alerts that they’re starting to feel annoyed while sending broadcasters so many viewers they’re likely to keep livestreaming. Facebook is even paying media organizations (including Fortune parent company Time Inc.) to produce live videos.

Facebook Live directly competes with Periscope, Twitter’s livestreaming video app. Twitter acquired Periscope last year, and many view the deal as a success. On Twitter’s first quarter earnings call Tuesday, Twitter CEO Jack Dorsey responded to a question about the threat from Facebook. Here’s his answer (as best as I could transcribe):

We’ve been doing live for ten years. We believe we have leadership position in it. It’s not just about showing the live event, it’s about hosting the conversation around the live event. …

As we talked about last time, we think the easiest way to get what Twitter is, is to show people a live event–things you can’t find anywhere else—and connect people and encourage them into the conversation. That is exactly what we’re focused on.

Because we are public, distributed, and simple we’re the fastest way to see a live event unfold, but also, [once it’s on Twitter], it can go everywhere.

We’re working on making sure we have the best experience out there and using technology appropriately to increase that experience and the enjoyment of the experience.

With Periscope, we think we have a significant leadership position in livestreaming video that is the best for broadcasters and their fans.

It’s true that Twitter has a lead when it comes to “live,” but that lead isn’t guaranteed to last. Facebook may not be able to imitate the speed and immediacy of Twitter, but with 1.6 billion users, even a “good enough” product is a viable threat.