Google ramped up the pressure on Facebook last night, announcing that MySpace and Bebo had joined the broad alliance of social networking sites it has assembled that will offer a cross-site platform for software developers.

The decision by MySpace, which has 110 million users, and Bebo - the most popular networking site in the UK - to sign up to Google's initiative, called OpenSocial, is likely to put pressure on Facebook to drop its own, similar, platform and join up too.

OpenSocial will provide software developers with a common set of standards, meaning that their programs - or applications - will be compatible with a range of social networking sites.

Facebook opened its platform to developers in a similar way in May, and since then more than 5,000 applications or 'apps' - including the popular music tool iLike - have been written for the site and downloaded by millions of users.

"OpenSocial is going to be become the de facto standard for developers right out of the gate," Chris DeWolfe, chief executive of MySpace, said during a press conference at Google’s headquarters. "It will have access to 200 million users, making it way bigger than any other platform out there."

Google said that Facebook had been asked to participate in the platform, but the latter, which now has 50 million users, has so far declined to comment, saying only that it will evaluate OpenSocial once it's had a chance to look at it.

"The most important principle about openness is that everyone is invited to join," Eric Schmidt, Google's chief executive, said.

Other networks that have signed up to OpenSocial include Friendster, Hi5, and LinkedIn, and a number of companies which traditionally provide software for businesses, such as Salesforce.com and Oracle, have also joined.

"The internet has fundamentally changed the expectations that consumers have for business applications," Adam Gross, vice president of developer marketing for Salesforce.com, said, adding that companies would increasingly deploy social applications for their customers.

Dan Nye, chief executive of LinkedIn, a network aimed at professionals, said: "Some social experiences are about photo sharing, some are affinity-based, some professionally oriented. OpenSocial allows each experience to be distinct and provides them a shared infrastructure for development."

Google will be hoping that the initiative generates renewed interest in its own social network, Orkut, which, while popular in Brazil, has struggled to gain a foothold in the US.

It may also benefit by selling more advertising on networks that join OpenSocial, as well as inside applications written by third party developers.

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