Apple has become the second company in the world worth more than $300 billion. Meanwhile, less formal estimates put Facebook at $50 billion.

It was only last May that Apple overtook Microsoft, with a market capitalization of $221 billion. That figure represents a company’s current stock price, multiplied by the number of stocks in the company. In effect it’s the paper value of the company and the amount it would theoretically cost to take it over completely. (In effect, somebody offering to buy the whole thing, or Apple stockholders all deciding spontaneously that they wanted to sell, would likely distort the stock price.)

The company hit a new milestone when stock markets opened for 2011: at a stock price of $329.60, it’s new valuation is $302.56 billion.

The only real target left — and bear in mind that this is simply paper value, rather than the hard cash of revenues and profits — is to overtake current number one Exxon Mobil at $368 billion, which would either mean the oil giant slipping, or Apple getting a hair over $400 a share. That’s about a 20% further rise, which is a bit of a stretch but could be possible if the company either has some great revenue figures, or if any of the big product announcements this year strike a chord with investors.

Meanwhile Facebook has been valued at $50 billion, though that’s even more of a paper estimate. The logic behind the figure is simply that a report state that Goldman Sachs and a Russian investor have paid a combined $500 million for one percent of the company. Of course, that means the entire valuation is based on a single transaction, so it’s far beyond pinch of salt territory.

Ironically there was said to have been a rule of thumb in the dotcom boom of the late 90s/early 2000s of valuing tech firms at 25 times their annual revenue. The report of the Goldman Sachs deal puts Facebook’s revenue at $2 billion, which works out bang on the money. (Then again, it may not be a coincidence, but rather that Facebook and Goldman Sachs used this method to come up with a “fair price.”)

But while the headlines are about how Facebook now has a higher value than eBay or Time Warner, the real story appears to be that financial regulators are asking whether such stock deals — which will likely see the shares passed on to Goldman Sachs clients — have effectively created the type of market in Facebook stock that is meant to be restricted to Wall Street and other regulated exchanges.