But you don't have to do that, any more than an invading army has to occupy all centers of power at once. And people aren't going to spread across zillions of decentralized forums because people put value on network effects, and larger networks are worth more than small ones.

Thus Facebook isn't one giant blob of people yelling at each other, but has huge numbers of groups where people can meet, while also being able to find/contact almost anyone else. Of course I participate in small decentralized forums relevant to my specific interests/hobbies, but I don't only watch those, and you probably don't either. That would be like only ever reading local news and skipping news about your state/country/international events. You can do that but you'll be putting yourself at a big disadvantage, which most people prefer not to do.