(BusinessWeek) - All good things must come to an end, and so it is with SecondMarket Inc.’s four-year run as the biggest private market for buyers and sellers of Facebook Inc. (FB) (FB) shares. The initial public offering Facebook has planned for today forced an end to the private trading, which has driven the company’s valuation up more than 10-fold since 2008 to exceed $100 billion. Commissions on Facebook trading may have accounted for almost a third of SecondMarket’s revenue last year.