Facebook fans can now buy shares for under $20 apiece, little more than half the May initial offering price of $38.

But they should probably hold off. Here's why.

Earlier this year, SmartMoney.com used revenue projections to calculate that Facebook was worth anywhere from $9 a share to skeptics to $15 for bulls (see "A Safe Price to Pay for Facebook"). Wall Street has adjusted its revenue forecasts since then, so here's a fresh look.

Pricing a young, fast-growing company is complex work, but investors can reduce it to a manageable job by combining Wall Street's math with a couple of simple judgments.

Step 1 is to focus on revenues rather than earnings. It's easier to predict the gross amount of money a company will bring in than what will be left over after subtracting for expenses, investments, taxes and so on.

Step 2 is to leave the work of predicting revenues to the army of analysts employed by Wall Street, and the smaller army of pollsters who turn individual estimates into consensus forecasts. These forecasts might not prove accurate, especially ones for two or three years from now, so investors should view the conclusions that come from them as best guesses rather than gospel.

Step 3 is to select a mature dotcom company to use as a benchmark. Google (GOOG) is a good choice.

Step 4 is to imagine what Facebook shares will look like as they shift from the high valuation of a young company to a more Google-like valuation. Google trades at 5.1 times its revenue from the past four quarters. The median for S&P 500 companies is just 1.6.

Google fetches a higher price-to-revenue ratio than the market because it turns a high percentage of its revenue into profits, generates piles of cash and is growing its revenue nicely -- 35% last quarter versus a year earlier, or 21% excluding its acquisition of cell phone maker Motorola Mobility.

Facebook trades at 9.8 times its trailing revenue. A skeptic on the company might say that it's only worth Google's 5.1 times revenue. That implies a market value of around $22 billion, or $10 a share.

But Facebook is growing its core revenue faster than Google -- 32% last quarter. Investors who wish to factor in this faster growth should look to future revenue projections. For example, Facebook is projected by analysts to bring in $8.2 billion in revenue by 2014, versus $4.9 billion this year.

To turn that into a stock price, consider that Google trades today at 3.6 times its projected 2014 revenue. Assuming Facebook will generate the same investor enthusiasm as Google in two years, investors should pay just under $30 billion today, or about $14 a share.

To price in even more expected growth, look to Facebook's 2016 forecasts. But the math again works out to $14 and change.

Two, offsetting factors have changed since we looked at these numbers in May. Revenue projections for Facebook have fallen, but the price for Google has jumped with the broad market. The broad conclusion is pretty much the same: The safe price to pay for Facebook stock seems lower than today's price.

There's another, short-term reason to be cautious. Facebook's recent price decline is owed in part to early investors being freed to sell their stakes. That process will continue through early next year, so the selling pressure might continue.

By the way, LinkedIn (LNKD), another social network stock, has bucked the trend of initial offering flops among dotcom companies. Shares were offered to premarket investors at $45 apiece in May 2011 and closed their first day of trading at about $94. They sell for $104 now.

Careful there, too. The company's revenue is expected to more than triple through 2016. But factoring in all of that growth, and applying a Google-like valuation to the 2016 forecast, the price bulls should pay today works out to about $68 a share.

For Facebook and LinkedIn alike, either the revenue forecasts are too low, or the share prices look too high.