I don't understand why you think a distributed social network couldn't work without web adverts? There are plenty of organisations that would derive value from setting up nodes. Universities would. I'd imagine it coming as a free addon to email services and other subscription services.
If centralization is required for a service to grow to a large size, how do you explain email? If email was being designed from scratch today, we'd have people asking why anybody would set up a free to use email service.
"Universities" was an example of one type of provider. Setting up a "free to use" node would be useful even for commercial organisations. Think brand recognition. You'd also have charitable organisations doing it for the sake of it. I don't need to supply an exhaustive list to show that it's easily possible.
