Latest Facebook Messenger 25 user agents:

User Agent Platform Device
Mozilla/5.0 (iPhone; CPU iPhone OS 7_1_2 like Mac OS X) AppleWebKit/537.51.2 (KHTML, like Gecko) Mobile/11D257 [FBAN/MessengerForiOS;FBAV/25.0.0.4.14;FBBV/8936291;FBDV/iPhone4,1;FBMD/iPhone;FBSN/iPhone OS;FBSV/7.1.2;FBSS/2; FBCR/OrangeFrance;FBID/pho iOS 7 iPhone 4S
Mozilla/5.0 (iPhone; CPU iPhone OS 7_1_2 like Mac OS X) AppleWebKit/537.51.2 (KHTML, like Gecko) Mobile/11D257 [FBAN/MessengerForiOS;FBAV/25.0.0.4.14;FBBV/8936291;FBDV/iPhone3,1;FBMD/iPhone;FBSN/iPhone OS;FBSV/7.1.2;FBSS/2; FBCR/NOVA;FBID/phone;FBLC/ iOS 7 iPhone 4
Mozilla/5.0 (iPad; CPU OS 7_1 like Mac OS X) AppleWebKit/537.51.2 (KHTML, like Gecko) Mobile/11D167 [FBAN/MessengerForiOS;FBAV/25.0.0.4.14;FBBV/8936291;FBDV/iPad2,5;FBMD/iPad;FBSN/iPhone OS;FBSV/7.1;FBSS/1; FBCR/;FBID/tablet;FBLC/fr_FR;FBOP/1] iOS 7 iPad Mini