About a month ago, Messenger implemented an in-app browser, similar to the one on the main Facebook app. It's terribly slow, and I'd love to load links in Chrome (or Tumblr, or Twitter, or whatever app is actually meant for the link). I've looked all over and cannot figure out how to disable it - all help I find is for Facebook's in-app browser, not Messenger's. I'm using a Oneplus 3 with the default Oxygen OS. Thanks!
Not sure if you figured this out or not, or maybe they fixed it, but I figured it out. Open messenger. In the far upper right corner, tap the silhouette icon that takes you to your profile. Scroll down and check "open links externally"
While this is a feature of the Facebook app itself, it appears there is no feature to do this with the current version of Facebook Messenger as of this answer (version 126.96.36.199.61).
The only current method in order to use the Android assigned default application for the URL is to Click the Menu Icon and click Open in ...
However if you are noticing that your in-app browser is slow, you may need to head into your Settings -> Apps and clear cache for Android System WebView and Google Chrome. (You may have to turn on Show System in the three dot menu.)