noted..so if if just want to forward the RTMP stream to FB using ffmpeg, could I just use options - c:v copy and -c:a copy?
Please clarify the case: do you want to publish a stream to WCS from webcam, then republish this stream to a third party server, then publish the stream from third party server to Facebook using ffmpeg?
If yes, the common response is yes, you can republish the stream to Facebook as is. But this depends on your third party server which accepts RTMP stream republished from WCS.
You can also republish WebRTC stream as RTMP directly
from WCS to Facebook (if this is not locked in the country where server instance is placed of course).