@Alan, I had the same message and it is due to customers still playing old builds of my game. Some over a year out of date!
And customers using Android API < 15 which can no longer update.
I thought that might be the case.
Would still be handy to know which version of the api the plugin is currently using, just for future reference. Any chance that someone at Corona could add that to the docs for the plugin at some point?
Go to https://developers.facebook.com/tools/api_versioning/ and select your app. It should show you the affected endpoints.
I always just get this message:
Your app hasn’t made enough calls to the Graph API to show any info for the methods you selected between v2.3 and v2.9. Visit our changelog to see the full list of changes in all versions.
I get that on any app that I click on, which isn’t very helpful. I’m sure it will be fine 
It looks like we are using 4.16.1. Based on the date of the plugin, it looks like we should support up to v2.8 (v2.7 worst case) of their API calls.
When Facebook kills an API version, your API calls will still go through, they just get redirected to API endpoints of the lowest supported versions. In other words any v2.3 API calls will be forwarded to v2.4 end points. Now your mileage may vary. Facebook doesn’t change all of their end points on each release so some or potentially all of your calls could continue to work. Since we are using a modern enough SDK this shouldn’t be an issue anyway.
Rob