And that’s because Facebook’s 1 billion daily active users aren’t just logging onto Facebook to check in with friends and family. For many users, Facebook is a de facto storage system for photos, the first place people find news and information, a private chat platform, and the way to log into countless other sites across the web.

There are, however, two distinct fissures in Facebook’s otherwise total dominance. One is among the very young, who opt for Snapchat and Instagram over Facebook; and the other is among the professional crowd, many of whom may be using Facebook but also spend an enormous and increasing amount of time plugged into Slack. Seventy-seven Fortune 100 companies now use Slack, the company says. (The Atlantic, like many newsrooms, has used Slack since 2014.)

As Slack rapidly grows, its approach to keeping users in one place increasingly looks like Facebook’s. The same way that Facebook doesn’t want you straying into the wilds of the open web to read a news article that one of your friends posted, Slack doesn’t want you heading over to Tumblr to search for the perfect reaction GIF for your Slack chat. So where Facebook has Instant Articles (which allow users to read outside articles from within Facebook), Slack has integrations with companies like Riffsy, which offers a smattering of GIF options when prompted by a Slack command (kind of like the Giphy command in Slack, but without the terrifying roulette quality). Both companies are building mobile-first products—which, of course, they should be.

Facebook is still far, far bigger than Slack. Facebook has more than 300 times as many daily active users—1 billion to Slack’s 3 million. But Facebook has also been around since 2004. (And remember, MySpace was bigger than Facebook until Facebook turned 4.) Slack’s growth so far is, by any measure, astounding.

Does this mean that Slack might someday be the new Facebook? This isn’t an unreasonable leap to make; just stay with me for a second. Though it seems that the two sites are for fundamentally different purposes—one for work, one for socializing—there’s reason to suspect that a Facebook-Slack power struggle isn’t out of the question. (And not just because of all the other ways work and non-work bleed together, culturally, especially in the United States.)

Slack isn’t built for social performance in quite the same way that Facebook is, but Slack does seem to be more a reflection of the real-time web than Facebook. Yes, Facebook has Messenger and News Feed, but there’s still something about the interface that feels old-school—at its core, Facebook is more like a bulletin board than a conversation. Perhaps more importantly, for people who are seeking a place to convene with small (ideally private) groups of friends and family online, Slack could be preferable. And there’s evidence that people are increasingly seeking such social environments online. (Again, see Snapchat.)