Several names come into mind when I think of online services offering developers the tools to "create" their own social networking sites from piggybacking off their framework(s). One which has been in the media lately is Ning. From my experience with them, their services and frameworks do seem to give developers the ability to do interesting things; but the question I have is why?
As of yet, I haven't heard of any recognizable brands (Nike, Reebok, Walmart, Coca-cola, etc) who have successfully created a site that has gone critical mass. Yet even with this fact, social networking sites are still in demand by clients who genuinely feel that owning a social networking site is a viable business opportunity which they should invest in; they want to be the next Facebook.
Far be it for me to say that social networking sites are a poor investment decision, but an interesting fact about the web: Ashton Kutcher has almost 5 million subscribers. He doesn't own a website, let alone a social networking site. And as far as I know, I don't recall any recognizable brands like Walmart having even 1 million subscribers (website, RSS feeds, Twitter feed or otherwise); let alone 5 Million.