Facebook Messaging App
The app in question — Facebook Messenger — is an extension of Facebook’s existing messaging system, which allows two Facebook users to communicate within the Facebook platform. As a standalone program separate from the Facebook app, Messenger allows Facebook users to send text messages between phones, while still storing the conversation in a user’s messaging center on his or her individual Facebook page.
Facebook’s Messenger app is also capable of group texting, letting you pick multiple Facebook contacts and send them all texts at once. If a FB user is logged into the new message app, it’ll show up there. If not, they get the message sent to them as an SMS, so long as they’ve registered their phone with Facebook. The service is mobile-focused, so it doesn’t completely integrate with Facebook Messages, which tries to route a message to a person using whatever channel Facebook thinks is most likely for you to get it fastest.