Another reason for investors to avoid getting linked up with LinkedIn: the pile of stock options outstanding. At $93 a share, the company is worth $8.9bn. But factor in options and, assuming today's share price, the valuation jumps 17% to $10.4bn.

That level of new shares, which will dilute existing shareholders, looks excessive even by Silicon Valley standards, where companies have long attracted employees with generous equity compensation. At Google, for instance, the impact is about 1%. It was roughly 6% when the company went public back in 2004.