Hearthstone has exactly this system (only friends can chat with each other). What happens quite often after some matches: 1. You receive a friend request from your opponent 2. If you accept it, they probably are going to send you a bunch of insults and then unfriend you.

In my case with my son, the guy who blackmailed him to give gifts was a friend of a friend of a friend. So usually you have only 2,3 people you know in real life, then each one will have other 2,3 and they will bring them into a group to play together. I never bought Fortnite shit for my son so he had no stuff to give and he got reported multiple times, probably some shitty employee that is super bored will randomly click some buttons to empty his queue.