by Andy M. Zaky, contributor

Like many privately held companies, Facebook is very tight-lipped about its financial performance. It told us it became cash flow positive for the first time in September 2009, and earlier this summer it announced it had eclipsed 500 million subscribers. It continues to push into new businesses – earlier this week it announced new location-based features – but when pressed to share the plans for monetizing these businesses, Facebook CEO Mark Zuckerberg typically declines to elaborate.

But all this mystery doesn’t stop rampant speculation about Facebook’s valuation. Nor does it stop big investors from taking sizable stakes in the company in the hopes of getting handsome returns on an IPO that some insiders suspect will happen in 2012.

Based on a recent study released by eMarketer, Facebook is expected to bring in roughly $1.3 billion in revenue in 2010, nearly double the $665 million the research firm estimates Facebook recorded in 2009. Yet despite its enormous revenue growth, Facebook currently only brings in a meager $0.56 per 1,000 page impressions compared to the industry average of $2.43, according to Comscore. Furthermore, according to current estimates provided by Second Shares, Facebook makes only about $2.60 per user on an annual basis, which is significantly lower than the $18 made by Google (GOOG) or the $12 made by AOL (AOL).

And while Facebook is poised to surpass Google in terms of visits – in July, according to Compete.com, Google had 3.161 billion visits and Facebook had 3.152 billion — it’s worth questioning the company’s ability to fully monetize its user-base. But it’s also important to remember that other Internet-based companies started in a similar way — including Google — and that Facebook could easily improve upon its anemic revenue per-user growth.

Facebook insists there’s no imminent public offering. But that won’t stop us from asking: What is Facebook really worth, and what kind of IPO valuation might we expect?

Shares of Facebook already trade on two private exchanges, where a small market exists for investing in venture-backed companies. The trades aren’t made public, and the lack of liquidity makes it difficult to determine a true market value. According to Next Up Research, investors were valuing Facebook at between $11.1 billion and $12.5 billion earlier this year, based on an analysis of shares purchased on the SharesPost private exchange. Today, they’re valued at $24.9 billion, according to Bloomberg.

And, according to Larry Albukerk, a specialist at EB Exchange Funds who privately brokers shares of Facebook, the company occasionally trades at an even higher valuation. “There are very larger, sophisticated institutional investors who are buying at a $30 billion valuation,” he recently told MSN Money.

The volatility of the private market

Those are big swings, but Facebook investors are all too familiar with such volatility. When Microsoft (MSFT) took a $240 million stake in the company in October 2007, it was valued at $15 billion – the same valuation it had in early 2008 when Hong Kong billionaire Li Ka-Shing made the second of two $60 million stakes. But by 2009, Facebook’s value had dropped. A $200 million stake made by the Russian technology firm Digital Sky Technologies in May 2009 put the company at a valuation of roughly $10 billion.

So are private investors getting overzealous in their assessment of the company or will these large stakes prove as lucrative as they were for Google’s earliest investors? With 500 million subscribers, Facebook already owns a quarter of the world’s Internet users. Yet, as the financial community learned with YouTube, having a gaggle of users is only one part of the equation.

Facebook will probably be able to monetize its user-base more efficiently in coming years as its business strategy shifts, says eMarketer analyst Debra Aho Williamson. Although half of Facebook’s current growth comes from the blockbuster success of its self-serve ad platform, its future lies with big-brand advertisers who want to reach customers through Facebook and are willing to pay higher CPM rates (cost per thousand page impressions) than the current platform delivers. Procter & Gamble (PG), the world’s largest advertiser, continues to take a significant interest in Facebook, and other big brand names will likely follow.

Secondly Facebook will soon see a significant uptick in user-growth through international markets, which is key in making the overall platform very attractive to brand advertising.

Finally, despite the company’s massive customer base, it’s still far outpacing Google’s growth in users. According to recent estimates, Facebook grew its user-base by 150% in 2009 versus Google’s 40% growth based on similar metrics.

Even if Facebook doesn’t substantially raise its revenue per user in the immediate future, that staggering user growth by itself justifies a valuation of nearly $50 billion over the next several years. Facebook is expected to earn nearly $1.8 billion in revenue in 2011 and that’s based on a projected 600 to 700 million users. Google currently trades at a $150 billion market capitalization and the only thing standing between Google and Facebook is Google’s revenue per user. If Facebook figures out a way to command similar revenue per user rates as Google, the company could potentially be worth upwards of $150 billion.

It almost doesn’t matter exactly when Facebook’s business model meets its full potential – it is certain to have a welcome reception whenever it decides to go public. What investors will likely see on Facebook’s IPO is a rally not seen since Google’s IPO. Many investors missed out in getting ahead of Google’s meteoric stock surge, and Facebook will give investors a second opportunity to participate in a blockbuster IPO.