We’re still wondering how and why it’s legal to keep selling shares privately after registering for an initial public offer. Three auctions of Facebook shares have occurred since the company registered with the Securities Exchange Commission, and they’re occurring more frequently than ever.

The last one took place just six days after the previous one, the shortest time between auctions. The most recent one closed yesterday on SharesPost, putting Facebook’s implied valuation at $98.7 billion to $105 billion, depending on how many outstanding shares you assume exist.

SharesPost’s website goes with the higher number, which would mean 2.5 billion outstanding shares. The lower number means that Facebook has 2.35 billion outstanding shares. We’ve been seeing both share counts out there for the last five auctions so we have to cite both.

If in fact Facebook has quietly issued more shares since the last auction, that would explain why the stock closed down $2; most likely the sellers in the latest round were eager to capture profits from the stock reaching a new record high in the last auction.

But are these private auctions of stock occurring after an IPO registration something that lawmakers should be concerned with?

The New York Times’ Dealbook suggests that companies such as Facebook and Twitter have essentially funded private market exchanges like SharesPost, accounting for up to a third of their revenues, in some cases.

These private exchanges exist mainly to help employees and early private backers cash out some of their holdings. While Facebook has proven to be a boon for these markets, the activity has drawn scrutiny from the SEC.

In fact, Facebook is working with the SEC to formulate new rules over how these exchanges are governed, according to the IPO registration documents. The social network rightly wants to exert more control over who owns their shares and how employees’ trade them.

The SEC is also considering changing a rule that says private companies with 500 or more stakeholders must publicly disclose its financial results on a quarterly basis. The agency is considering raising that number.

In a letter signed by SEC chairman Mary L. Schapiro to the powerful chairman of the Government Oversight and Government Reform committee Darrell E. Issa, Schapiro confirms that the agency is looking more closely at secondary market exchanges. Issa initially wrote to Schapiro with concerns that SEC rules were inhibiting companies from going public.

Schapiro wrote:

The staff also is currently monitoring the secondary trading activity on a variety of online trading platforms, many of which are facilitating the trading of securities of private companies. Trading that develops on online trading platforms can be beneficial in that it can provide much desired liquidity to investors, which can assist in attracting investors to smaller private companies. This benefit, however, must be balanced with investor protection concerns that can be raised when there is a lack of information available to investors about these private companies.

Here’s’ a copy of the email that went out to SharesPost members around 7:30 pm Pacific Time last night:

SharesPost Financial Corporation completed its auction on February 14, 2012 of 200,000 units of an investment vehicle designated to hold shares of Facebook, Inc. (“Facebook”). A clearing price of $42.00 per share was established at this auction.

Members submitting qualifying bids at or above the clearing price will be contacted shortly with instructions on next steps for completing this transaction. Successful auction bidders will hold an indirect interest in the shares of Facebook, Inc. through their ownership of units of an investment vehicle designated to hold the shares. The administrator of the investment vehicle, SP Investments Management, LLC, is a wholly-owned subsidiary of SharesPost, Inc.