When most of us first heard of it a few years ago, Snapchat seemed like a cheesy-sounding service whose primary feature—the ability to have photo messages disappear automatically after 10 seconds—made it sound like nothing more than a tawdry “sexting” app for teens.

How times have changed.

That cheesy-sounding app now has a theoretical market value of about $20 billion, thanks to the devotion of its growing base of users, who reportedly number more than 150 million a day. That makes the company larger than Twitter (TWTR), according to some estimates.

And Snapchat has gradually expanded its service to appeal to a larger proportion of the video-sharing crowd, to the point where it gets about 10 billion video views a day.

The latest step in that expansion is a new feature the company is calling “Memories,” which allows users to save photos and videos and look at them later. In other words, Snapchat just introduced a feature that essentially reverses its original value proposition, which was the ephemerality of messages that automatically self-destructed.

Like the company’s Discover section, where users can browse news from media partners such as CNN and BuzzFeed, the new Memories feature may make Snapchat more appealing to an older user group. But this also raises the prospect that the service’s hard-core millennial fans might get turned off by the new additions before enough older users sign up.

Ironically, Facebook (FB) launched a feature of its own this week that is somewhat similar to Snapchat’s: Users in a new test will be able to save videos for offline viewing on the giant social network. And the goal of both features is likely similar—namely, to boost engagement levels to appeal to advertisers (which Snapchat is also doing).

Snapchat’s video vision:

But there the comparison stops, at least so far. Facebook has already more or less justified its $330 billion market valuation with revenue of $20 billion last year. But Snapchat likely had revenue of about $100 million in 2015, with promises of as much as $300 million this year.

The company’s ability to produce that kind of growth is going to determine whether it’s actually worth its $20 billion valuation, or whether that’s going to disappear the same way its photos and videos used to.