It's a matter of differentiating between a) what we recognize as the real earnings potential (which some see as fairly limited), and b) what John Q. Sharebuyer and Joe C. Fundmanager will probably do (buy buy buy, it's the interwebs!). The reality of 'b' means that shorting is not wise.
I don't like linkedin's current valuation - but I think shorting it in front of several very high profile tech IPOs would be foolhardy at this point.