I've just noticed that this method is failing in my ASP.NET web application because I'm not catering for 'expires_in' being missing in the response.
I haven't changed any code recently, and was not aware of any reason for this to be missing in the response. Does this mean that the token has expired?
I'm using version 7.0.6 of the C# Facebook SDK and v2.5 of the Facebook Graph API.
The JSON response looks like this:
{"access_token":"xxxx","token_type":"bearer"}