I think Anil Dash hit it on the head, in his Now I get it! post (though perhaps he merely is being sarcastic?):

In the future, everyone will have his or her own social software application, and then try sending out invitations to their friends to have them join it. Everyone's worried about the services being centralized, but with five billion of them sprouting up, we'll be completely decentralized. It'll be just like our email address books!

I actually think individuals will each have many social network sites, not simply one. This is how I have imagined social networking as a feature of the iCite net: essentially a mechanism of interoperability and trans-location between networks / sites, any of which may have social features.

Interoperability, in this context, is the idea that any networks / sites should be able to interconnect their information and their functionality. You should be able to have a list of friends on one site, and use that as an access control list on another site, for example. Or, a view of your address book should appear as the blogroll on your blog.

Trans-location (which is a term I just grabbed out of my hat, and is meant as a more general term for transclusion) is the idea that any networks / sites should be able to include other networks / sites. So, you and your spouse can have a shared site that includes portions of your personal sites, for example.

Trendy social networking sites are, at best, like trendy bars (or other public spaces): you and your friends can decide to hang out there. At worst, these social network sites try to trap you in their spaces.

But, I assume, even when everyone can create all the social sites / spaces they want, some spaces will end up being more trendy than others and people will want to go there. In fact, if people aren't trapped, they will probably "bar hop", as I started to suggest in my Orkut: social network and/or online cruise ship post.

(In that vein, a bar where everybody knows your name is going to be fun if it has a few dozen people [< 150 tops?], and social networking sites with thousands of people aren't going to be exciting except in the more Woodstock / Burning Man / spring break = brief, but extreme, largely anonymous, adventure kind of way. In fact, the value of such sites may be greater as their social happening is briefer and more anonymous!)