Google has thrown down the gauntlet to Facebook with plans to compete with the social network’s hugely popular software development platform.

The devilishly cunning but widely expected move - coming just a few days after Facebook’s strategic partnership with Microsoft – will see the online ad giant allowing developers to build apps and syndicate them across different social media sites.

Dubbed OpenSocial, it will presumably vie with the APIs of Facebook and Google’s ad partner MySpace – both of which are proprietary – and will be seen by app companies as a great opportunity to scale up.

According to Reuters, it has already received support from “about a dozen” social networks, including LinkedIn, XING, hi5, Friendster and Google-owned Orkut, while successful software makers such as iLike and Slide are also involved.

iLike chief executive Ali Partovi told the news agency:

“For months we’ve been approached by other websites that want us to build iLike widgets for them and we’ve been unable to build it for them. The benefit OpenSocial offers us is we can essentially … syndicate what we do to other social networks.”

Compatibility between social networks’ development platforms has been seen as a growing issue for app makers, especially after MySpace’s announcement that it was launching its own API earlier this month.

Developers will be keen to see whether MySpace and Facebook react by offering cross-network support.

On OpenSocial, GigaOM writes:

“OpenSocial attacks Facebook where it is the weakest (and the strongest): its quintessential closed nature. Several Facebook developers have groused that a special Facebook-only mark-up language makes the task of writing Facebook apps tougher.”

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