http://www.businessweek.com/technology/content/nov2008/tc200...
The second market in Facebook is definitely pushing the envelope, a role of the SEC is to make sure people don't get ripped off and are treated fairly.
Clearly promoters shouldn't be able to sell private unvetted securities to the public without registering and going IPO on a public exchange. But it's also not clear it's in the public interest to have second markets only the well-heeled have access to, and that are not subject to public securities regulation, public price discovery and disclosure through K and Q filings. Maybe it's also not right to force Facebook to choose between going public prematurely and issuing shares/options to employees, but at some point for those shares to be properly valued the stock has to be publicly traded.
When they got the exemption they said it was just to give shares to employees, not to create a second market. The rules are getting rewritten (or Wall Street is running rings around them as Kedrosky says).
I fundamentally like the second market idea a lot because I like private companies staying private for as long as they want while still giving employees options for ownership that are at least somewhat liquid. It's really fascinating to see the mechanisms involve creaking at the joints a little, but I think the idea is solid.
In any case, the company presumably has a right of first refusal clause on their stock, which means that Facebook can match any stock buyer's offer themselves, or broker a trade to someone else instead. That's only in case of emergencies, though.
A lot of other companies did so around that time, since this was following the accounting standards change which required options to be expensed. If you're going to expense options, you might as well expense something more flexible.