The issue with JS SDK has been resolved recently according to the Status page: https://developers.facebook.com/status/issues/492652922187445/
Update · UTC
Facebook is currently working to fix JS SDKs issue - you can keep track of this incident on their Status Page here: https://developers.facebook.com/status/issues/492652922187445/
Investigating · UTC
We're receiving reports that Customer Chat plugin is not working as expected, disappearing from websites for certain customers.
We will update this incident once we have more news on the issue.