This morning the company threw its support behind OpenSocial–a Google-led initiative to foster interoperability between social applications–and with MySpace (NWS) and Google, it announced the OpenSocial Foundation, a nonprofit dedicated to the mitigation of perceptions that Google will use OpenSocial for its own benefit.

“… Today … we’ve joined forces with Google and MySpace to create the OpenSocial Foundation, and will also begin supporting the OpenSocial standard,” Yahoo VP of Platforms Wade Chambers said in a post to Yahoo Anecdotal. “Industry consortiums such as this often start slowly and evolve over time. So far, OpenSocial is rapidly growing and adapting, but still in the early stages. We feel that this is the right step at this stage in its evolution. It’s no longer a trial balloon–it’s for real. We are taking this opportunity to help ensure Web sites and developers feel confident using OpenSocial as the building blocks for their new social apps.”

Queried by the Times, Yahoo offered no further detail on the rumor. “Yahoo has a rich history of supporting open standards, such as OpenID and Apache Hadoop, as we believe industry collaboration is beneficial to the developer community and the Web as a whole,” said a company spokeswoman. “While we are evaluating OpenSocial as an emerging standard, we do not comment on speculation or rumors.”

Anyway, if this particular rumor proves true, it’s potentially big news. As Forrester Research analyst Charlene Li notes, it “would mean that the site with the largest group of users, and with the largest base of registered users, would be joining OpenSocial.”

“[We] want to share the benefits of our work by enabling other social sites to use our platform architecture as a model,” Facebook senior platform manager Ami Vora said in a blog post announcing the move. “In fact, we’ll even license the Facebook Platform methods and tags to other platforms. Of course, Facebook Platform will continue to evolve, but by enabling other social sites to use what we’ve learned, everyone wins–users get a better experience around the Web, developers get access to new audiences, and social sites get more applications. “

Among the first sites to avail themselves of the Facebook Platform standards was Bebo, an OpenSocial partner. Isn’t that a bit of a slap in the face for Google? Bebo co-founder and CEO Michael Birch says no. When OpenSocial finally launches, Bebo will support it as well. “OpenSocial and the Facebook Platform are clearly different platforms,” Birch told News.com. “Our lazy development team said they couldn’t do both at once.”

Bebo and Six Apart are nice additions, but it’s MySpace that will bring quite a bit more heft to this effort–an addressable market of some 70 million active monthly users, at last report.

Who’ll next join OpenSocial? Will it be Facebook? CEO Mark Zuckerberg’s likely puzzling that question out at this very moment as he waits for an invitation to join–which apparently hasn’t yet been extended. “Despite reports, Facebook has still not been briefed on OpenSocial,” the company said in a statement. “When we have had a chance to understand the technology, Facebook will evaluate participation relative to the benefits to its 50 million users and 100,000 platform developers.”

OpenSocial, as Google has named it, is a set of common APIs (application programming interfaces) that will enable developers to write applications for a broad range of Web sites and services without any individual customization. Think of it as Facebook CEO Mark Zuckerberg’s “social graph” but writ large.

And while some might smirk at OpenSocial’s initial roster of participants–LinkedIn, hi5, Ning, Friendster, Plaxo and Google’s own “big in Brazil” social network Orkut–it does include a few big names: business software makers Salesforce.com and Oracle. Oh, and Google. Which, as TechCrunch’s Erick Schonfeld points out, already has much of the critical mass it needs to push this effort forward: “Google already has so much data on you, depending on how many Google apps you already use. It just needs to bring everything together. … Over time, Google will connect all of these together in different ways, along with data about you from other social services across the Web, and give developers access to the social layer tying all of these apps together underneath. The real killer app for Google is not to turn Orkut into a Facebook clone. It is to turn every Google app into a social application without you even noticing that you’ve joined yet another social network.”