facebook messenger for IOS 6.1.6

Probably doesn't support that version of iOS. Most apps are updated for the latest version, and supports maybe an OS version one generation back. You could potentially find an older version of the app, but it would require your phone to be jailbroken most likely, and the app is likely to be prone to errors/crashes because it is defunct. Best bet is to just update your iOS version.
 
Aug 5, 2013
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well i dont know about ios not being supported as i dive have in on there fine from march till a few days ago