I don't see how you can compare this to the housing bubble/crisis. The real estate market contains trillions of dollars (everyone's money participates in it one

I don't see how you can compare this to the housing bubble/crisis. The real estate market contains trillions of dollars (everyone's money participates in it one way or another), and some of the worst stuff, like bad mortgages, was amplified and leveraged by firms using it as collateral. Here, we're talking about a $9 billion valuation for LinkedIn, and that's just a valuation, they sold off something like 20% of the shares, so actually only $1 billion or $2 billion changed hands. In other words, the leverage here works to reduce the economic impact. Who cares about this, other than the people holding these shares? The percentage of the world's capital being invested at these insane valuations is very, very small. Meanwhile, there are tons of undervalued companies in everything from shipping to construction and energy. I think we're not looking at a depression just yet, at least not based on a few dumb valuations for no-substance "social" companies.