Facebook’s new mobile messaging app is similar to Snapchat. Slingshot stands out by requiring you to reply to a message before you’ll be able to open the message.
So if you were to send your friend a message, they’d have to send you their reply before reading what you sent.
Slingshot — now available on the iOS app store and Android Marketplace — is an attempt to emulate the feeling of writing a letter. Instant messages are easy to ignore, so the app makes you earn the right to open them.