Facebook is rolling out Poke, an iPhone app that makes it fun to say hello to friendswhile on the go. The poke feature has been a part of Facebook since 2004. With the new Poke app, you can poke or send a message, photo, or video to Facebook friends to share what you're up to in a lightweight way. You can poke an individual friend or several at once. Each message expires after a specific time you've set, either 1, 3, 5 or 10 seconds. When time runs out, the message disappears from the app.

To see a message from a friend, press and hold the message until it expires. If you ever see something you're uncomfortable with, you can click the gear menu and report it.

Facebook will make available Poke in the App Store starting later today.