Zynga (ZNGA), the world's largest social gaming company, had a rather disappointing public debut, shedding nearly 5% of its stock price on the first day of trading. It competes primarily with other social gaming companies like Electronic Arts (ERTS) and Playdom, which was recently acquired by Disney (DIS). We have a $10.20 price estimate for Zynga, which stands just above the offering price for its IPO. New games account for more than 40% of its Trefis price estimate.

Zynga has made some really popular games like CityVille and FarmVille, which had a user base of close to 100 million at their peak. However, social games can get old fast and user numbers can drop as quickly as they grow.

CityVille's user base is now down to less than 50 million, while FarmVille has around 30 million active users. Most of the games Zynga launched in 2011, like Empires & Allies and Adventure World, peaked early and are already showing a significant decline in their user base. Zynga also launched a sequel to the very popular Mafia Wars -- Mafia Wars 2 -- but it failed to gain significant traction and its user base has declined to less than 7 million.

Zynga desperately needs another mega-hit, like CityVille or FarmVille, to continue its explosive top-line growth.

One recent games, CastleVille, may be the answer to its short-term problems and could help support its stock price. CastleVille, which was launched a month ago, is adding millions of users every week and is now officially the fastest growing game on Zynga.

As Zynga's older games lose active users, we expect new games like CastleVille to show a steady growth and account for an increasing portion of its revenue.

Check out our complete analysis of Zynga.