Belatedly recognizing the importance of social networking as an online service and advertising vehicle, Google Inc. 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 (application programming interfaces) 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 websites, 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 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.

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 US$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.