"Wikipedia is effectively one-of-a-kind. No other mass-market or topically broad wikis have had meaningful success to date. Even Wikimedia's other wiki projects are not nearly as active as Wikipedia. If successful wikis are rare, Wikipedia might be a one-in-a-million lightning strike — some unique combination of factors succeeded in this case, but those circumstances are unlikely to replicate. If so, Wikipedia's rarity might also highlight its fragility."