Latest Facebook Messenger 23 user agents:

User Agent Platform Device
Mozilla/5.0 (iPad; CPU OS 8_2 like Mac OS X) AppleWebKit/600.1.4 (KHTML, like Gecko) Mobile/12D508 [FBAN/MessengerForiOS;FBAV/23.1.0.15.15;FBBV/8176219;FBDV/iPad4,4;FBMD/iPad;FBSN/iPhone OS;FBSV/8.2;FBSS/2; FBCR/;FBID/tablet;FBLC/ru_RU;FBOP/1] iOS 8 iPad Mini 2
Mozilla/5.0 (iPad; CPU OS 7_1 like Mac OS X) AppleWebKit/537.51.2 (KHTML, like Gecko) Mobile/11D167 [FBAN/MessengerForiOS;FBAV/23.0.0.14.15;FBBV/7829425;FBDV/iPad4,2;FBMD/iPad;FBSN/iPhone OS;FBSV/7.1;FBSS/2; FBCR/Verizon;FBID/tablet;FBLC/ru_RU;FBOP/1] iOS 7 iPad Air