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system
can a bunch of wealthy people force facebook to go public?
user
@alex b's answer hits most of it, but leaves out one thing: most companies control who can own their non-public shares, and prohibit transfers, sales, or in some cases, even ongoing ownership by ex-employees. so it's not that hard to ensure you stay under 500 investors.   remember that sharespost isn't an exchange or clearinghouse; it's basically a bulletin board with some light contract services and third-party escrow services.   i'd guess that many of the companies on their "hot" list explicitly prohibit the sale of their non-public shares.
assistant