Belatedly recognizing the importance of social networking as an online service and advertising vehicle, Google this week will unleash its strongest response yet to the rising threat and skyrocketing popularity of MySpace and Facebook.

On Thursday, Google will announce OpenSocial, an initiative with a set of APIs at its core that is 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 Web sites, simplifying the making and distribution of these programs.

Partners supporting OpenSocial include various well-known software developers and social-networking sites: 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 Web site 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 catalog.

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.

Along the way, social-networking sites have been created for other purposes beyond entertainment and staying in touch with friends. For example, there are social networks tailored for specific occupations, such as doctors, and interests, such as sports.

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.

With about 300 employees now, Facebook expects to have about 700 a year from now, its CEO and co-founder Mark Zuckerberg recently said. About 250,000 new users register every day at Facebook, which was founded in 2004.

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.

More information about OpenSocial and resources for developers will be available from Google later this week. There will also be a developer sandbox.