Well, if you do happen to want another option, you might be pleased to learn that Facebook now allows you to dedicate an entire browser page to only Messenger, simply by hitting up Messenger.com and clicking the big blue button to log in.
As the shot above highlights, this website is optimized for any platform, and could prove useful for those who simply refuse to install the Messenger app. As far as I can tell, both the mobile app and this webpage version look nearly identical, and in quick tests, it works well.
I could see this solution being useful to those with multiple monitors, who need to keep conversations active all day long. You can simply dedicate an entire webpage to the messenger, and put it to the side. It looks much better in my opionion than the built-in messaging system on Facebook, and it actually scales, so you may very well consider it to be a great option, despite the fact that so many others already exist.