"That said, I think the argument for a 'paid facebook' breaks down quickly when you realise that as a communications / event planning / promotion tool, it's usefulness is directly tied to its network size."
I generally agree with the business analysis as you gave it, to which I'd propose the idea (in conjunction with my other cousin post) that if the government makes it non-viable to build a business whose business model is based solely around selling private information and serving ads , on the grounds that it is short-term enticing to people but long-term destruction to social capital and cohesion (where government regulation is really in its wheelhouse), we could find out whether social networks are viable at all.
Note I didn't target social networks there; the entire business model needs to be made much more expensive. For once, something regulation can accomplish with ease! I almost don't even care about the nature of the regulations, I just want to see this business model made more expensive. It doesn't even have to be banned, per se, it just needs the bar raised so that the bar for "when is it worth it to abuse trust and exploit our information" is a great deal more than $20/year. (I don't necessarily want to ban it; there is value in certain high-end variations of this theme. The corruption is in the way that modern computing has become so cheap that there is no discretion applied for what is grabbed and what is bought & sold, it's just all grabbed and dispensed freely.)
It is possible they are simply not a viable business. Or, once the easy-yet-culturally-destructive option of doing it by abusing human cognitive limitations and screwing over your customers where they can't perceive it and weigh it properly is regulated away, perhaps someone will find a way of delivering enough value that it is something people will pay for, once the weed of the free option is no longer sucking up all the sunlight. There are, for instance, quite a few long-standing paid forums on the internet; it's not an entirely unviable business model.
One thing I'm not doing here is that I am not implicitly accepting as a goal the idea that Facebook itself, or an entity the size of Facebook, will necessarily exist in the new order. (Nor am I directly accusing anybody of doing that, but it's something I feel is ambient in the discussion.) If it turns out to be simply a non-viable business, so be it. As in my opinion Facebook's net social value is negative, that's still a win. If it turns out to be a viable model, but it doesn't scale up, again, so be it. The possibility that those will be the end outcomes does not justify keeping something as destructive as Facebook around.
My best guess is that no single business could grow to Facebook's size, but that there would likely be a steady state of a lot of little networks of various kinds. And don't just think 25 Facebook clones, but thing Facebook vs. LinkedIn vs. the SomethingAwful forums vs. Reddit; I would expect significant diversity.