After a rocky start, Facebook’s life as a publicly-traded company has settled down a bit. Despite healthy skepticism about its prospects in light of a $100bn valuation, its stock price has stabilised (for now) while industry observers and investors are taking stock of its actual — not hypothetical — business.

But one of Facebook’s best friends, social gaming giant Zynga, hasn’t been so fortunate. Today, its stock plunged to under $5, its lowest level since the company went public last year.

Sparking the sell-off in Zynga shares is a report that the company, which is behind popular social games like FarmVille and CityVille, saw its active daily users on Facebook decline by a notable 8.2% in the past month.

For obvious reasons, that news has spooked investors who are not only wondering whether Zynga’s games have hit a slump, but whether social games on Facebook have jumped the shark. At least one analyst believes it’s the latter, writing “we now additionally believe that interest in Facebook-based gaming may have reached a negative inflection point.”

But is the news really so bad? Chances are if you ask Zynga, it’s not.

After all, Zynga has been trying to diversify. Like other social gaming companies, it’s moving aggressively into mobile gaming, which is a potentially far more lucrative market. And for its web-based games, it built, which it hopes will also become a Facebook-like platform for other gaming firms. If Zynga is successful in making sure that all its eggs aren’t in the Facebook basket, it will naturally derive less and less from the world’s largest social network.

The immediate problem, of course, is that the rapid, precipitous decline in its Facebook audience appears to be occurring so quickly that it’s necessary to ask: if Zynga makes less money from Facebook as a result, are its non-Facebook investments paying off quickly enough to offset the loss in Facebook revenue? The answer may very well be no, and with the investments it will need to make in its own platform and mobile to remain competitive likely to be substantial, the economics of Zynga’s business may be in for a dramatic change in the near future.

For all of Zynga’s well-documented faults, its rise from nothing to billion-dollar powerhouse in a few short years is remarkable. Arguably, one of the reasons it was able to grow so quickly, and successfully navigate the challenges that come with nearly-unprecedented growth, was that it was a private company and had the flexibility to break all the rules. Now that Zynga faces challenges that will realistically require it to make big changes, it’s not clear whether it will be so capable as a publicly-traded company.

Which may prove to be the biggest lesson in this latest .com IPO boom: going public as a billion-dollar company can, somewhat suprisingly, be a lot easier than remaining a billion-dollar publicly-traded company.