LinkedIn shares more than doubled in price in their first day of trading, valuing the annoyingly pointless social network at more than $8 billion. Now that the social networking bubble has reached the masses, it isn't just annoying, it's dangerous.

From an offering price of $45, LinkedIn shares rose as high as $122.70 and have now settled down to around $110. Why is this insane? Well, to justify present worth of roughly $8 billion, LinkedIn will not only have to keep gouging corporate and business customers with access fees, but gouge far more of them, "from a few thousand customers today to tens of thousands." Yet users aren't visiting the site as often as LinkedIn would like, probably because it is clogged with annoying, friend-spamming careerists no one wants to deal with and because, unlike with Facebook, there's not much reason to come back to the site once you've set up a profile.

Until now, the nutso nosebeed overvaluations of companies like Twitter and Facebook were problems only for the wealthy investors able to get a piece of the privately held startups, and perhaps for the occasional pension fund . Now that LinkedIn has become "NYSE: LNKD," it's a problem that will be shared by mutual funds shareholders, individual public stock investors and, potentially, anyone depending on the good fortune of said equity holders. Welcome to the beginnings of systemic risk.

[Chart, top, via Yahoo Finance]