2. Zynga

Social gaming company Zynga ( ZNGA) is the closest way to invest in Facebook without owning the shares.

Facebook generates a significant portion of its revenue from Zynga (approximately 11% of 2011 revenue and 12% of first-quarter 2012 revenue), and is so important to Facebook that it was listed as a major risk factor in its S-1 filing. "If the use of Zynga games on our platform declines for these or other reasons, our financial results may be adversely affected," the S-1 said.

San Francisco-based Zynga has been diversifying itself away from Facebook, launching its Zynga.com platform and there are rumors that the company is expanding into online gambling. Zynga recently bought Draw Something maker OMGPOP for $200 million to diversify beyond Facebook.

Shares of Zynga have lost 11.37% this year.

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