For most of the past year, Facebook has been the hottest playground for developers to build applications that allow millions of people to play an online version of Scrabble or throw sheep at one another. But thanks to Google, MySpace may soon be the new virtual hotbed for developers.

Last week News Corporation’s MySpace (NWS) became the first social network to adopt Google’s (GOOG) OpenSocial, a set of common standards that allow developers to build applications. Since then, some developers who have been spending virtually all their resources building for Facebook’s open platform say they are now shifting a majority of their efforts to Facebook’s chief rival.

“We’re spending 90% of our resources building on OpenSocial now,” says Jia Shen, the CTO and co-founder of RockYou, a prolific widget maker that has built popular Facebook apps like SuperWall and Horoscope. “For our company, launches are really, really important in establishing an initial footprint.”

Says Keith Rabois, vice president of business development at rival widget shop Slide: “We are investing an incredible amount into MySpace.”

Though MySpace officially opened its doors to developers on March 13, developers complain that company hasn’t actively promoted the new applications to make users aware of them. They also say that MySpace, which has been plagued by privacy and spamming issues in the past, is being overly cautious in rolling out its open platform. “It’s like being handed a cell phone and not being allowed to make any calls,” Shen says. “We’re still waiting for the chance to grow and spread our apps on the site.”

MySpace did not return a call or e-mails seeking comment.

Still, the potential to tap a massive audience is enough for hundreds of developers to shift their attention to the MySpace platform. While Facebook is steadily gaining users, MySpace remains the largest social networking site in the U.S. Last month MySpace boasted 68 million unique visitors with the average user spending four hours on the site, according to comScore.

Google, whose own social network, Orkut, is little known outside Brazil, has been eager to make itself relevant in social networking. After Facebook popularized the open platform for social networks, Google announced its OpenSocial initiative last fall to encourage other networks to do the same using Google’s common set of standards. Among those that have joined the Google alliance include Orkut, LinkedIn and Hi Five, which will roll out its OpenSocial-padded platform next week.

“This was a brilliant idea on Google’s part,” says Shayan Ghazizadeh, the founder of Zoosk, which makes a popular dating app on Facebook. “They could have sold everyone to use the Orkut platform. Instead they set up this independent standard and encouraged everyone to gang up on Facebook.”

OpenSocial director Joe Kraus insists OpenSocial was never about Google going up against Facebook. Over a cup of soup last week at the Googleplex, Kraus discussed Google’s longterm vision. “The notion of having your friends around with you across the web will happen,” he says. “The notion of being social on any site will occur. You should be able to go to eBay and see what your friend’s friend has reviewed. What we want is the ability to have your friends to become social on the web.”

Google says any site, not just social networks, can choose to use OpenSocial. No permission needed. Yahoo is reportedly planning to use the OpenSocial standard for some of its web properties. Bebo, the social networking site that AOL bought last week for $850 million, had originally aligned itself with OpenSocial but has since switched to using Facebook’s platform.

Kraus says he would “absolutely love” for Facebook to join OpenSocial. “Part of the nice thing about open governance is that all these [social networks] can have influence,” he says. “You can better align interests, and all your partners feel like they have a stake. What’s the famous line? ‘If you want to go fast, go alone. If you want to go far, go together.’ ”

Facebook will likely try to go fast and far on its own. It has little incentive to convert to OpenSocial given that it has its own platform in place. Not to mention the fact that one of its minority investors is Microsoft (MSFT), which is clearly not a fan of Google.

Even without Facebook, OpenSocial is getting the backing from developers it needs on MySpace, according to Kraus. He says OpenSocial is something developers can “learn once” and “write anywhere.”

But some developers argue that the technical challenges of writing for different platforms isn’t the problem. Because every network has its rules, quirks and culture, using a common standard doesn’t necessarily save developers time. “Customizing the features takes a lot of work,” Slide’s Rabois says. “What you develop for MySpace users is different from Facebook users or Bebo users. This isn’t driven by technology. At the end of the day, it comes down to the users.”

“The reality is, you have to do special things for every social network,”Ghazizadeh says. “Maybe it’s not the same amount of work, but there’s still a lot you need to do. Google’s not the savior that it wants to position itself as.”

Perhaps not, but then, that was never Google’s intention with OpenSocial. “Truthfully,” Kraus says, “The goal is to make OpenSocial part of the fabric of the web.”