Mafia Wars, Zynga can handle. Market wars, maybe not so much.

The second-quarter profit dip that the San Francisco game maker reported last week will probably delay its initial public offering until November, after second-quarter results sparked concerns that growth is slowing at the producer of games such as FarmVille and Mafia Wars, according to analysts at Renaissance Capital and IPO Boutique.

But Zynga's value may have dropped as low as $10 billion from $20 billion estimates this summer, said Paul Bard, a vice president at Renaissance. The differences: an 11% drop in the Nasdaq since business-to-business social network LinkedIn (Nasdaq: LNKD) doubled on its first trading day in May, and a 92% drop in Zynga's second-quarter profit, driven partly by Zynga's first-ever quarter-to-quarter decline in bookings, a measure of new sales during the quarter that includes some purchases that won't be recognized as revenue until later. Bookings dropped 4%.

"The $20B figure was thrown around back when (Zynga) filed and everyone was going bananas for the big Internet 2.0 names," Bard wrote in an email. "But with the markets down 15% and given the 2Q bookings number, that seems like an aggressive target if it were to price the deal today."

I had come to a $10 billion valuation for Zynga, too. The company made $177.3 million before interest, taxes, and noncash expenses in the first half of the year, down slightly from last year. In its key metrics, the number of players on Zynga's games each month has stayed around 230 million for more than a year, and the amount each user spends per day on virtual goods like electronic "poker chips" and "farm animals" used in Zynga's games has recently stayed at $0.05 to $0.06 per day.

On these numbers, $10 billion works out to about 25 times last year's EBITDA, generous considering that EBITDA is down so far this year. TripAdvisor, the travel-review site Expedia (Nasdaq: EXPE) is spinning off this year, did $166 million of first-half operating income before amortization, and all of Expedia is worth just $7.1 billion. TripAdvisor is growing profit faster than Zynga this year, with first-half operating income before amortization up 23%. All of Expedia commands just 23 times TripAdvisor's OIBA today.

The market's dip reminds us that even Internet companies don't branch out much. Instead, they remain rooted in the businesses they disrupt: Blue Nile is a jeweler, Expedia is a travel agency, and so on. Zynga is a very strong gaming company, much like Electronic Arts (Nasdaq: ERTS) or Activision Blizzard (Nasdaq: ATVI). EA, maker of Madden NFL, is worth about $7 billion even as it commands a perfectly respectable 18 times estimated earnings. Activision, which makes World of Warcraft, is worth about $13 billion at 13 times earnings, and TakeTwo Interactive (Nasdaq: TTWO) is worth $1.1 billion at 66 times this year's estimates. None has seen shares rise since early 2008.

Gaming is a tough, hit-driven business. And while Zynga has done better than many at making a succession of hits, the sheer difficulty of the task is why gaming companies usually don't get super-high valuations. And it argues against giving Zynga a market cap -- like $20 billion -- that assumes sustained success before it happens.

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