Facebook have resolved the issue. Thanks for your patience.

Facebook have deployed a fix. They continue to monitor the situation. https://developers.facebook.com/status/dashboard/

Customers may be experiencing issues with replying back to users on Facebook Messenger.
Please note that Facebook is experiencing a partial outage at the moment. More information is available on their status page: https://developers.facebook.com/status/dashboard/
We have no open incident on our end but will be monitoring the situation.