There is a lot more to an investment than just the valuation. If VCs are shooting for outsized returns on home run investments that make up for their failed investments, then Greylock shouldn't have made the investment at all. "At least $1 or 2 billion big but no one had certainty it would end up being $50-100 billion+ big" - that sentence is laughable in its tautology. If people had certainty about that, the valuation would be way more than $500MM and another $250MM would have made no difference. For VC fund to invest at a $500MM valuation in hopes of an exit at $1-2B, they have already indicated that they are riding the coattails of earlier investors. This article makes it sound like Greylock evaluated Facebook in an intellectual bubble and took a big leap of faith, but that's just what they want you to believe.