Hello, your question is way too vague. 

1) What version of Pega and Customer Service application are you using

2) Has your Facebook channel been configured to create a service request? Or is it just conversational? 

3) Are only external users seeing issues? Or did you find the same in your testing? 

Hello,

I am preparing for CLSA Exam and I need to know how to address that issue. This question was asked and I could not find any material on PDN regarding this topic.

If the complaint is about creating a customer service request, that would mean that the channel is configured and the connectivity is good. I would test the case creation directly in the Channels landing page. If it works there, that means the conversational version of the flow is working. If there are still problems, the next step would be to trace the fb HTTP service directly and try. This would be hard in production, since it captures every customer's request. You would have to do this in Dev or QA instance so you can be sure that only one person is hitting the service. 

Thanks Amit for providing troubleshooting steps!