Latest Facebook Messenger 27 user agents:

User Agent Platform Device
Mozilla/5.0 (iPod touch; CPU iPhone OS 8_1_2 like Mac OS X) AppleWebKit/600.1.4 (KHTML, like Gecko) Mobile/12B440 [FBAN/MessengerForiOS;FBAV/27.0.0.34.13;FBBV/9902566;FBDV/iPod5,1;FBMD/iPod touch;FBSN/iPhone OS;FBSV/8.1.2;FBSS/2; FBCR/;FBID/phone;FBL iOS 8 iPod Touch 5
Mozilla/5.0 (iPhone; CPU iPhone OS 7_1_1 like Mac OS X) AppleWebKit/537.51.2 (KHTML, like Gecko) Mobile/11D201 [FBAN/MessengerForiOS;FBAV/27.0.0.34.13;FBBV/9902566;FBDV/iPhone3,1;FBMD/iPhone;FBSN/iPhone OS;FBSV/7.1.1;FBSS/2; FBCR/Free;FBID/phone;FBLC iOS 7 iPhone 4