Hi Tim,
Thank you for posting, I am here to help.
Messenger application has been discontinued. You can still use the web version by checking this link: http://messenger.com/
Since the functions is same with messenger desktop app. This is the web browser version of your facebook messages.
looking forward to helping you out. If you need more help please let me know. If the answer is helpful, please click "Accept Answer".
Best regards,
Jansher