Zynga is trialling ‘reward advertising’ on CityVille that allows players to refill their energy bar by interacting with an advertiser.

Sponsors involved in the trial, which began in December, include Coca-Cola, MasterCard and the 20th Century Fox film ‘What’s Your Number?’.

The ads appeared when a player was running low on energy – some were offered the option of watching the film trailer or completing a MasterCard survey in return for a boost.

Zynga’s main source of income is virtual currency but it has offered players rewards for similar brand interactions before. In June FarmVille players were given ‘Double Mastery’ points if they placed a Capital One statue on their farms.

Though stats haven’t been released, this newer rewards trial was obviously positive, since Zynga plans to roll out similar ads to FarmVille and Empire & Allies soon.

Zynga has been investigating other ways of diversifying its revenue streams recently, even touting a possible move into online gambling.

While it is a profitable business, there is some uneasiness around the fact that even though it has 152m monthly users, only 6.7m of them provided its revenue during the first nine months of 2011. This means that in theory, only 4.4% of Zynga’s users think the games are worth paying for.

The value of Zynga’s freemium games was given a boost in the documents filed for Facebook’s IPO.

Zynga contributed $445m to Facebook’s $3.71bn in sales last year, which equates to 12% of the social network’s revenue, and the IPO documents state that Facebook’s financial results would be harmed if it fails to maintain its business relationship with the gaming company.

The millions of dollars come from the 30% cut of transactions Facebook takes from in-game purchases and from money Zynga spends on its advertising platform.

The filing cites a study which says the virtual currency market will generate $15bn in 2014, up from $7bn in 2010.

News of Facebook’s reliance on Zynga led to a near 20% jump in the gaming brand’s stock price on Thursday.