Is there anyway that the ones who watched but didn't report be identified, They should be all arrested and charge for being accessory to rape.
I'm not sure about how police really get info through Facebook but they probably contact them, FB is not able to know who's watching live videos but who knows, they might keep the data for themselves. If that's the case, It could be possible to retrieve those data but I doubt they will provide them to the police as this will only prove that they are collecting info about users behind their backs.