You can't create a community
There are lots of companies trying to cash in on this stuff. But building a site that provides a personal profile, blogging tools, private messaging, and whatever other features won’t create an online community. It can only facilitate it.
I don’t believe it’s possible to create a community. A community has to form.
Furthermore, a community can only form around a particular point of commonality.
LinkedIn, for instance, is for business people who want to stay connected as their careers develop.
MySpace gathered steam as a site for people in a certain geographic area and age range who were interested in a certain range of music. It spread geographically, but the demographic didn’t change much. (I was teasing a friend of mine that he was now on MySpace, and he acknowledged that it was only because his new, much younger, girlfriend was on it.)
Being fellow members of the human race isn’t a particular point of commonality. Even if we get invaded by space aliens, there would be multiple human communities: one for those who want to band together and fight, and one for those who want to cut a deal. There needs to be something around which a sense of community can form.
Being an owner of a Putt-Putt Motors vehicle isn’t such a thing, but being an avid owner is. This is a good reason for Putt-Putt to support an avid-owner community (one probably does exist, even if most people think Putt-Putt’s vehicles are horrible), by providing an online space for instance, while avoiding any attempt to create a broad “owners’ community”. (I said “support” for a reason: an owners’ club, in order to be that and not a “manufacturer’s club”, cannot be seen to be controlled by the manufacturer.)
No particular point of commonality, no community. I believe that every general-purpose social-networking site is doomed to failure, unless it’s lucky enough to have one or more strong community groups spring up within it that are sufficient to carry it.