Although these sites vary widely in their original goals, they all have one idea in mind - creating links to other people, which supposedly mirror relationships you have with them in other aspects of life. These relationships supposedly mean something to you, as well: whether they be business, personal, or online relationships, they each have a significance attached to them of some kind, supposedly, otherwise you would not be creating them.
These sites all fail at the idea of mapping relationships in a way that even vaguely represents reality. The reason for this is simple: these links are entirely artificial, based only on the statements of a person when creating their "profiles" for these sites. Quickly, these things become competitions - how many friends can I have? How big is my 6-steps network? They then become boredom killers - hm, I wonder if it's possible to traverse my network to get to $RandomPublicallyKnownFigure. In the end, the members of these sites typically allow the relationships they've artificially created to lapse, because there is nothing to do.
There are some exceptions to this rule, however, in general, a site that identifies itself as a "social networking site" isn't. I don't know if you believe this or not, but Marc Canter does not truly have 755 personal friends. Not everyone who lives in Japan is only two steps away from shaking hands with Joi Ito, and I can tell you flat out that if I had to invite 92 people to a party, the list would not include some of the people that I list as friends on Orkut.
In the end, Social Networking sites - sites where the goal is to create and build artificial relationships - mean nothing. They are not a decent indicator of who you know or have met, they are not an indicator of how popular you are or aren't, they are not anything other than a toy that some people like to play with more than others. They can not solve world hunger, nor can they help you to create that strong business team that you've always wanted. They can't do any of this because they don't represent relationships. They don't represent anything other than someone being a bit more excited to click a button than other people are.
Social networks exist. They exist as a result of day to day interactions with other people. They are built via emails, via instant communications, via phone calls and talking. They are built as a result of the combination of all these things. They are not created because someone clicks a button on a website. Just because someone labels themselves as your friend, this does not mean they are. Social networks are things which are built in interactions, not in websites. And until we can accurately map those interactions in a way that machines can understand - and understand the whole of it, not just parts of it - without any human interaction, social networks will not indicate anything more than what I just described - someone eager to click on a button.
I didn't use to feel this way. Orkut started as a way for me to keep track of contact details for people who I really considered close friends. But then the wave of requests came. "You're my friend, aren't you?" How can one say no to that question? To most people, you really can't. So my network grew. And grew. No longer is it really just my friends, no longer is it people I consider myself to be a participating member of a social network with.
Social networking sites are useless. Because they are useless, they become stagnant pools of dated information used only to further the egos of the few who feel that they are the "way of the future". I'll admit that that I used to feel that way. Now, I know better.
Social networks are important. Social Networking Sites are not.