In the latest example of brand advertising integrated into social games, Microsoft ran an advertising offer for Bing within Zynga’s hit game FarmVille on Tuesday. If users became a fan of Bing’s Facebook Page by clicking on a sponsored ad on the bottom right of the FarmVille main page, they’d receive 3 Farm Cash (FarmVille’s virtual currency). The effort was apparently successful, as Bing’s page went from slightly more than 100,000 fans on Monday to more than 500,000 as of earlier today.

FarmVille has 83.1 million monthly active users, and 28.7 million daily active users on Tuesday, according to AppData. So only about 1.5% of all FarmVille users participated in the 24 hours that the campaign ran. This engagement number is pretty good, especially if you’re a page administrator looking for a quick way to get more fans, or, say, a search engine trying to educate the public about your alternative to Google.

Bing also made an effort to make the action relevant to the game’s players. The sponsored ad (here), which used SocialVibe’s brand engagement ad service, directed users to become fans of the Page, mentioning that “whether you want to buy a horse or a tree, Bing can help you decide!” Bing also posted a status update to its Page earlier today, reiterating the message — “Any FarmVille fans out there? Try using Bing to get the most out of your crops and animals.” It linked to Bing search results for “farmville animals,” encouraging players who wanted to look for more information about the game would use it instead of Google. As most Bing Facebook fans today came in through FarmVille, the update was heavily commented and liked.

Other forms of advertising within games have typically run within offer walls, which include direct payment options for virtual currency as well as advertising offers. The number of users who do any sort of payment or offer is generally low, from 1% up. But that stat typically reflects the portion of users over the lifetime of a game. This was just for one day.

It appears that users do in fact appreciate advertising, and offers in particular, when they’re easy to find and clearly valuable. FarmVille-focused blog FarmVille Freak noted the Bing campaign in a post yesterday, and polled its readers to see what they thought. The question was: “Will you be clicking the Bing FarmVille Sponsored Link?” The possible answers were “Yes, I love Farm Cash!,” “No, I hate Bing! Long live Google!” and “Maybe, would like the Farm Cash, not Bing!” So far, 68% of respondents, or more than 3,500 people, have vote for the first option. Granted, readers of a blog about a social game are probably more interested in Farm Cash than the average player, but still an interesting data point.

In any case, expect many more ads that integrate a brand goal into an incentivized action in a game. SocialVibe has already been running branded engagement ads in Zynga games, as we’ve covered; it’s currently providing all of the offers in PetVille’s offer wall, for example. Other companies are getting into providing branded offers, too, from established offer companies like TrialPay to newer ones like gWallet to traditional in-game advertising companies like WildTangent.