The Zynga IPO may be yet another way to buy Facebook and the future that is social networking.

Zynga is the biggest maker of games on the Facebook platform. Some of its biggest hits include FarmVille, CityVille, and Zynga Poker.  The company makes money through advertising and by selling virtual level-ups or gifts Facebook gamers can give to their friends.

The Zynga said it generates substantially all of [its] revenue and players through the Facebook platform and expect to continue to do so for the foreseeable future.

Indeed, every great platform not only benefits itself but also a slew of companies that joined the ride.  For example, Google spawned Demand Media (NYSE:DMD), a company that's now worth over $1 billion and Twitter spawned TweetDeck, which was bought by Twitter for between $40 to $50 million.

Zynga is the most successful company based on the Facebook platform. Regardless of how well the Zynga team itself performs from this point forward, Zynga's financial success will undoubtedly be tied to that of Facebook and social networking.

If Facebook gets obliterated by Google +, Zynga's going to tank. If Facebook continues to grow at a blistering pace, Zynga's going to benefit.

Zynga, therefore, is as good of a backdoor investment to Facebook as any.

What are other ways to invest in Facebook?

You can buy GSV Capital Corp (NASDAQ:GSVC), closed-end management investment company, which owns 225,000 shares of Facebook.  While that stake is a minuscule percentage of Facebook, it represents 15 percent of GSV's portfolio. 

The other options to invest in Facebook are London-traded Mail.ru (LON:MAIL), which owns 2.4 percent of Facebook, and Microsoft (NASDAQ:MSFT), which owns 1.3 percent of Facebook.  However, these two companies are large enough so that their Facebook investments are dwarfed by their core company assets.   

A final way to invest in Facebook is buying private Facebook shares through platforms like Sharepost, but this option isn't feasible for most retail investors.