The unregulated secondary markets for late-stage private companies like Twitter, Facebook, and LinkedIn are quickly becoming a big business for fund managers and even retail investors.

    Chris Sacca is one of the super-angels -- a group of seed-stage investors who have raised third-party capital to find the next Facebook or Twitter. Not as well known, however, is that Sacca also has raised more than $30 million from outside investors to invest in Twitter, even at this later stage of its life as a private company.

    The money is pooled into a pair of funds called Industry LLC and Lowercase RT, and backed by a pair of institutional investors.

    So far, Sacca has put all of Industry LLC to work and is almost done with Lowercase RT. Both vehicles are able to buy up Twitter shares from employees and preferred stockholders, according to multiple people with knowledge of the funds.

    Sacca also considered raising a third fund called 140 – going so far as to send out offering docs to a small circle of prospects -- but seems to have since changed his mind. He declined to comment for this story.

    Investing directly in secondary investments of private companies is not new. There are several firms devoted to such deals, and even some burgeoning marketplaces for accredited retail investors.

    What Sacca and others are introducing, however, are pools of capital raised to buy existing stock in a specific private company. Fortune has learned that funds have been raised by other managers to acquire shares in such companies as Facebook, eHarmony, LinkedIn and Zynga.

    One of those other managers is New York-based Felix Investments, whose funds are believed to hold approximately two million shares of Facebook stock, according to a secondary market source.

    Felix was launched last year by former members of Advanced Equities, a firm known for raising money to co-invest with venture capitalists in new issues. It's not exactly for the masses, but it's as close as one can get given accredited investor requirements.

How the pool works

    A typical Felix process works as follows: The firm identifies a company with a large number of buyable shares, and designs a fund. For the sake of example, let's say the target is Zynga. Felix then goes about identifying investors. Much of the interest is inbound -- particularly from Silicon Valley technologists -- but Felix also pays a secondary marketplace called SharesPost to blast its offerings out to select members. Both and Felix and SharesPost confirmed this relationship but both declined to discuss specific fund raising, citing SEC restrictions.

    Those who participate only know that they are getting a chance to buy into Zynga, the hottest social gaming company on the planet. They are not given any non-public financial data on the company, because Felix doesn't have any. Investors also are not told the price at which shares will be purchased. Instead, their money is put into an escrow account while Felix goes in search of sellers. Once Felix figures out the price, it goes back to investors and gives them a chance to fish or cut bait. Most usually keep a steady reel.

    Some other fund managers are more specific, offering either an exact price or relatively narrow price range. All of them, however, charge fees. This typically includes both a management fee and a carried interest. It's kind of like investing in a venture capital fund -- or even a traditional direct secondary fund -- except for the lack of transparency surrounding portfolio company financials.

    So why pay 2-and-20 (fees of 2% of assets and 20% of profits), or some variation, to a fund manager who is more of a middleman than someone who performs due diligence? Basically because such deals can be prohibitively expensive for retail investors to do solo.

    For example, say I had $250,000 that I wanted to invest in Zynga. I could go to SharesPost or better-known rival SecondMarket, and hope to find a seller match. But then there would be legal fees and other closing costs. Plus, Zynga would probably charge me a few grand to approve the share transfer. In other words, the bad bargain from a firm like Felix may be better than no bargain at all.

A sustainable market?

    Investors and fund managers interviewed for this story agreed that more and more of these funds will continue to be formed and funded. What seems more uncertain, however, is whether such a niche market can be sustained.

    Most of the big funds being raised are focused on just a small handful of companies: Facebook, LinkedIn and Zynga. Expect that Twitter will soon join that rarified air, but it's currently a difficult company to trade on the secondary market (Sacca is one of several people who has a special agreement with Twitter management, perhaps because he's a company advisor and was an early investor off of his own balance sheet).

    Each of those companies -- with the exception of Twitter -- would probably have gone public in most any other era. Maybe the financial meltdown got in the way, or perhaps there are simply a few too many iconoclasts like Mark Zuckerberg in the mix. Either way, it's not inconceivable that each one will have completed an IPO by this time next year.

    If so, what will firms like Felix look to acquire? For that matter, what do the secondary exchanges trade? Is there another group of multi-billion dollar start-ups that retail investors will buy without even a cursory look under the hood?

    My guess is no. This is a special purpose time. But that's okay for retail investors. After all, they can always buy shares on the Nasdaq.