In the US, a private company with less than 500 owners can dictate who can or can't become a shareholder (this is true in general, but I'm sure there are loopholes). Prior to Google's IPO I could not buy shares in Google at any price. The reason Google was "forced" to go public is the 500 shareholder rule. At a high level, with 500 shareholders the company is forced to do some extra financial accounting and they no longer can control who owns a share of the company, allowing me to purchase shares of google at that point. At that point, it typically becomes in the companies best interest to go public.
See this article about Google approaching the 500 shareholder limit in 2003.
Further, Sorkin is not quite correct that "securities laws mandate that the company go public" if by "go public" we mean list on a stock exchange, available for general purchase. Securities laws mandate what has to be reported in financial reporting and when you have to report it. Securities laws also can dictate restrictions on ownership of stock and if a company can impose their own restrictions.
A group of investors cannot force a company onto a stock exchange. If shares of Facebook are already for sale to anyone, then having >500 shareholders will force Facebook to file more paperwork with the SEC, it won't force Facebook onto the NYSE or NASDAQ. When that point is reached, it may be in Facebook's best interest to have an IPO, but they will not be required by law to do so.
Update: CNN article discusses likely Facebook IPO in 2012.
When companies have more than 500
shareholders, they're required to make
significant financial disclosures --
though they can choose to remain
private and keep their stock from
trading publicly. However, most
companies facing mandatory disclosures
opt to go public.
The Securities and Exchange Commission
gives businesses lots of time to
prepare for that milestone. Companies
have until 120 days after the end of
the fiscal year in which they cross
the 500-shareholder line to begin
making their disclosures. If Facebook
tips the scale this year, that gives
it until April 2012 to start filing