Google is finally set to unveil this week its response to the rising popularity of social networking sites MySpace and Facebook.

Belatedly recognising the importance of social networking as an online service and advertising vehicle, Google will make its own announcement on Thursday. OpenSocial is an initiative with a set of APIs (application programming interfaces) at its core designed “to spread social applications across the Web,” the company said.

These common APIs have been designed to let developers create social networking applications that will run on multiple websites, simplifying the making and distribution of these programs.

Partners supporting OpenSocial include various software developers and social networking sites such as Oracle, Salesforce.com, Hi5, iLike, LinkedIn, Slide, Ning, Friendster and Plaxo. Neither Facebook nor MySpace is on the list of partners at press time.

The move is also no doubt intended to dilute the massive success Facebook has had in attracting external developers since it opened up its website platform to third-party applications in May.

About 7,000 such applications have been built for Facebook so far, helping to make the social networking site more attractive to people and thus boosting its usage. MySpace recently announced it would follow Facebook’s example.

By allowing developers to write an application once and have it work in multiple social networking sites, the OpenSocial initiative could, in theory, water down the appeal of Facebook’s ballooning applications catalogue.

Once considered a fad of interest mostly to teens and young adults, social networks have significantly broadened their demographic appeal, attracting many people in their 30s, 40s, 50s and beyond. This is despite security and storage concerns about social networking sites.

Google has had a social networking site named Orkut for several years, but it doesn’t seem to have merited much attention from the company until recently, and only after the popularity and potential of this type of site became evident.

Google was reportedly locked in a battle with Microsoft over which one would be chosen by Facebook to buy a stake in the social networking company and earn a deal to provide advertising to it.

Microsoft eventually won, buying a 1.6 percent stake that values Facebook at an eye-popping $15 billion, although the social networking company reportedly will have revenue of just $150 million this year.

In June, MySpace had 114.1 million unique visitors worldwide, ranking first among social networking sites, followed by Facebook with 52.2 million, according to comScore. Hi5 ranked third with 28.2 million, while Friendster was fourth with 24.7 million. Orkut rounded out the top 5 with 24.1 million.