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The paper value of their stake is still 6.5M. The paper value of Facebook as a whole (the market cap) is now 20B higher than it was prior to that trade, if you take that share price as the new share value, which is what Forbes did, and what 37signals objects to.

Ah ok got it, cheers.

The last financing round valued Facebook at $50B, and now with this latest buy in of $6.5M it increased the valuation to $70B. That is where the $20B figure is originates.

That's the increase in the value of everybody else's stake. Since the stock traded at a higher valuation, they're presuming that increases the value of all the other shares too.