It took me a while to start using this feature in OBS.
For those who are having trouble transmitting to Facebook, I have adopted a very simple solution that yielded results: Just omit the Facebook server port in the nginx.conf file, like this: "push rtmp://live-api-s.facebook.com/rtmp/... "
Thus, the stream was received in the preview of Facebook.
Beside that, this solution allowed me to stop using recording settings to stream over UDP, since the RTMP stream are easily accessed through VLC.