(CNN) -- As expected, Facebook has launched a Snapchat-like mobile app that lets users trade ephemeral photo- and video-based messages.
But that's not what's most interesting about it.
With the new app, called Slingshot, you can't view an incoming message until you respond with a photo or video of your own. That's right: There can be no passive users on Slingshot, which is basically forcing its community to trade photos and video clips, quid pro quo, like schoolkids exchanging stickers or lunchbox snacks.
"With Slingshot, we wanted to build something where everybody is a creator and nobody is just a spectator," its creators said Tuesday in a blog post announcing the app. "When everyone participates, there's less pressure, more creativity and even the little things in life can turn into awesome shared experiences."
Judging by initial reaction, however, at least some users may find this stricture a little annoying.
"It's frustrating, not exciting when a friend sends you a shot and you can't immediately view it," wrote Ellis Hamburger in tech-news site The Verge. "Slingshot is a new and strange example of a messaging app that raises barriers instead of tearing them down, and increases the friction to viewing a friend's photo instead of reducing it."
In fairness, the app is designed not for sending urgent information -- we have texting or even phone calls for that -- but for exchanging spontaneous moments or snippets of creativity.
Facebook said Slingshot is available in the United States, starting Tuesday, on iPhone (iOS7) and Android (Jelly Bean and KitKat). A spokeswoman said the app will roll out to other countries in the future.
Slingshot, which leaked briefly last week before Facebook pulled it offline, is the social networking giant's latest attempt to compete with Snapchat, a rival messaging app that is popular with teens and young adults.
Snapchat is estimated to have more than 30 million users and reportedly turned down a $3 billion acquisition offer from Facebook last fall. Facebook later went on to buy WhatsApp, a messaging app that's especially widespread overseas.
Slingshot succeeds Poke, a messaging app that Facebook launched in 2012 and shut down last month after it failed to gain traction with users.
Slingshot messages are photo and video only, although users can superimpose bits of text on the images.
Like Snapchat (and Poke), Slingshot messages aren't meant to be saved. They vanish once the user responds to them or swipes them away -- a slight distinction from Snapchat messages, which by default disappear after a few seconds.
"Photos and videos that don't stick around forever allow for sharing that's more expressive, raw and spontaneous. We can connect the same way we like to live: in the moment," Facebook said in its post, which takes the unusual step of crediting its competitor.
"We've enjoyed using Snapchat to send each other ephemeral messages and expect there to be a variety of apps that explore this new way of sharing," it said.
Unlike Snapchat, Slingshot allows users to send, or "sling," messages to multiple friends at once. A push notification will appear when somebody sends you something, but you'll only see a pixelated image until you respond.