After making the biggest U.S. Internet debut since Google (GOOG), online game maker Zynga (ZNGA) debuted on the Nasdaq Global Market with an uninspiring performance as analysts are already casting doubt on its lofty valuation.

Zynga, which makes popular games such as Farmville and Words With Friends, opened up 10% at $11, but closed down 5.8%.  

The debut is disappointing compared with those of recent Internet IPOs like LinkedIn (LNKD), which doubled its share price in its first day but has since declined. 

The online game maker's IPO priced late Thursday at $10, coming in at the high end of its expected range. 

The IPO places a market valuation of $8.9 billion on the San Francisco-based company, well below expectations earlier in the year for a price tag of $20 billion. A November 2011 prospectus estimated the company was worth $14.05 billion. 

Still, Zynga raised $1 billion -- the most for a U.S. Internet company since search giant Google debuted in 2004 by raising $1.9 billion, according to Dealogic.

Wall Street and Silicon Valley will be watching Zynga’s stock closely to see if it suffers a post-IPO slump like the ones that have impacted other recent next-generation Internet stocks, including Groupon (GRPN), LinkedIn and Pandora Media.

“I’m waving a red flag,” Shah Gilani with Money Map Press, told FOX Business, pointing to upcoming competition from the likes of Electronic Arts (ERTS) and Walt Disney (DIS). “I would be very apprehensive about taking a piece of this at the IPO.”

Founded by Mark Pincus and Eric Schiermeyer in 2007, Zynga, uses social-networking sites like Facebook as a platform for its games, which include Mafia Wars, Café World and Zynga Poker. The company's prospects are extremely tied to Facebook, which inked a five-year agreement with Zynga in May 2010. 

“I think it’s probably risky to hop in after the gate,” Jay Pestrichelli of Zega Financial told FOX Business. “I think their products are great, but just because you like the products, doesn’t mean you like the stock.”

The IPO world is waiting anxiously for the IPO of social-networking king Facebook, which is expected to go public next year. Facebook is reportedly planning to raise a whopping $10 billion in its IPO and expects to garner an enormous market valuation of over $100 billion.

“I think there is a lot of hype in the social-media space and I think we might see that bubble blow off when Facebook finally” goes public, said Gilani.

Follow Matt Egan on Twitter @MattMEgan5