When Zynga (NAS: ZNGA) closed below $5 yesterday -- on reports indicating that players were flocking from desktop apps to mobile games -- it wasn't just a round milestone.

Zynga went public at $10 six months ago. It has spent most of its publicly traded life in the single digits as a busted IPO, but yesterday's new low was the first time that the social gaming leader closed for less than half of its original IPO price.

Zynga isn't alone. There are several companies that went public last year that closed at prices at least 50% below what underwriters managed to pull off during the IPO.

Let's check some of them out.

Source: Hoover's.

Horizon is a fledgling biotech working on treatments for arthritis, pain, and inflammatory diseases. Investors know going in that upstarts in biotechnology require patience, but that doesn't seem to be happening here.

KiOR's technology turns biomass into renewable crude oil. The company is just starting to generate revenue, but sluggish oil prices and investors rotating out of alternative energy plays have hurt this development-stage company.

FriendFinder Networks is the biggest loser on this list. Even though revenue inched higher at the online fling-starter in its latest quarter, a small operating profit is no match for the heavy interest payments on the company's $463 million in debt.

Jiayuan went public on the same day as FriendFinder. As far as dating websites go, China's Jiayuan has held up far better. Unfortunately, it's all relative here. A 57% drop is cruel on an absolute basis.

Some of these names may bounce back. In fact, Zynga and Horizon opened this morning at prices just above the 50%-off sale. However, the next time you're lamenting about last month's social networking IPO that soured, remember that things could be worse.

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