when webinar is full and redirected to social such as youtube what reports can zoom provide ?

Hey @Ali-Gaaly, you can look into the Webinar Reports here. You can live stream to YouTube Live, Facebook Live, or Facebook Workplace. 

Facebook, Youtube, and Facebook Workplace have live stream view reports that you can access on the account you're live streaming to

• Youtube Live stream’s metrics https://support.google.com/youtube/answer/2853833?hl=en
• Facebook Live video metrics https://www.facebook.com/formedia/blog/introducing-new-metrics-for-live-video

Assuming that's where you can find the metrics of the # of participants viewing your webinar on a live stream service.