Why do we care about Facebook? People you know and respect use it. That includes you. People you know and respect who scoff at it still know what Facebook is. Facebook, like Google, is popular enough to have become a verb as well as a noun. If the public ever got a crack at buying shares in it, lots of people would get rich.

That’s why mass clucking ensued among the technology press when the word came out Tuesday that Chief Financial Officer Gideon Yu is splitting. The Wall Street Journal, so far as we can tell, broke the news. It said:

The departure of the 37-year-old Mr. Yu and the ensuing search for a replacement are likely to renew speculation that Facebook is stepping up plans for a public offering, despite the rocky economy. The company, which has turned down several acquisition offers in the past, has said it is hoping to go public in the next few years.

But some employees and investors, who have poured roughly $455 million into the company, according to VC Experts.com Inc., are eager for Facebook to start planning an offering and have raised questions about whether it has enough money to sustain its growth. Many others have said the company is over-valued, which — in addition to the economic downturn — hampered its efforts to fund an employee-buyback program last year.

One person familiar with the matter said Facebook’s financials are strong and the company expects revenue in 2009 to increase at least 70% from last year. (The New York Times has details on that too.)

The Journal also referred to the now famous $240 million that Microsoft invested in Facebook, giving the service a perceived value of $15 billion (see No. 4 in our list above). The problem is, the WSJ reported, Yu’s job “has grown more difficult, as Facebook has struggled to raise additional money at lower valuations.” If Facebook revenue is supposed to grow 70 percent — a giant leap — history would suggest, and nearly insist, that last year’s revenue total would have been only enough to buy a pack of Smarties.