Facebook Messenger logo for iOS & Android.
|Initial release||9 August 2011|
126.96.36.199.42 (Android) (January 2, 2015[±]
Facebook Messenger is an instant messaging service and software application which provides text and voice communication. Integrated with Facebook's web-based Chat feature and built on the open MQTT protocol, Messenger lets Facebook users chat with friends both on mobile and on the main website.
Facebook reported in November 2014 that Facebook Messenger has reached 500 million users. David A. Marcus heads Facebook Messenger and had joined Facebook on invitation of Mark Zuckerberg, CEO of Facebook.
Messenger for Mobile
In December 2012, the Facebook Messenger app for Android in some regions (such as Australia, South Asia, Indonesia, South Africa, and Venezuela) added the ability to use Messenger without a Facebook account by simply using a name and phone number. These changes are intended to allow Facebook Messenger to compete against similar mobile messaging platforms such as WhatsApp as an alternative to text messaging. Later updates added the ability to use Facebook Messenger as a replacement text messaging client on Android, and added "Chat Heads", an overlay chat system originating from Facebook Home.
On July 3, 2014, a native Facebook Messenger app was released for iPad. It was specially designed for iPad rather than just running as an enlarged iPhone app. Messenger for iPad features a multi-window interface showing a list of threads and the current conversation at the same time. 
There is some controversy surrounding the permissions required to govern its functionality. Some of the permissions include such things as being able to send SMS messages, which may cost money.
Messenger for Desktop
Messenger for Windows Desktop
Messenger for Windows Desktop was officially released on March 5, 2012 for Windows 7 users. Testing on the software began among a limited beta tester group on November 21, 2011; however, a leaked link to the beta-stage software was publicly revealed to tech bloggers by Israeli blog TechIT. Facebook responded the same day by announcing the link's availability through its Help Center.
Among its features is a "ticker" similar in function to the sidebar of friend activity seen by Facebook website users. At the time of release, "chatting with multiple friends, video calling, limiting chat availability and editing settings" were not available through the software. A Mac OS X version was mentioned by Facebook developers as forthcoming.
Messenger for Firefox
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- Why Facebook Has Entrusted Its Future to the CEO of PayPal
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- Official website
- Facebook Messenger at iTunes Preview
- Facebook Messenger at the Google Play store
- Messenger on the Windows Phone Store.