This is a thought experiment, not a business plan, so don't expect too much here, but how hard would it be to set up something that worked along these lines?:
It would allow users to set up circles of contacts and permit different levels of access determined by user-assigned rank ("casual" contacts could see some pages while "close" contacts could see more), degrees of separation and visitor attributes ("let all visitors who list Justified on their favorites see this page);
The pages could be of any format and would be hosted by whoever the user chose as long as it used standard access protocols and had a few standard features like a favorites section;
Adding friends and updating settings would be done through a central site;
If run on a for-profit basis, revenue could come in through the central site with advertising and surveys (both targeted on user information), mining user data and selling apps for mobile access.
I'm not saying this is a good idea for a business (you'll notice I put it here and not on Kickstarter) but it certainly seems to fall in the realm of the possible and virtual social networks did start popping up they could conceivably eat away at the potential user base for Facebook.
You can (and probably should) object at this point that this is a somewhat farfetched scenario. You'd be right. You could also point out that Facebook is a good company with a sound business plan, loads of technical talent and a huge first mover advantage.
If we were talking about Facebook's chances of having a nice, profitable run those points would end this conversation, but the buyers in tomorrow's IPO are betting that the company will have a flawless, even unprecedented run. Under those circumstances, it's worthwhile to take a moment to think about the possibilities.