MySpace has agreed to join OpenSocial, Google's new platform designed to allow developers to build applications that will work on any website.

MySpace joins other sites including Bebo, LinkedIn and Orkut in signing up to OpenSocial.

'Next stage'

The participation of MySpace, which is the biggest network with more than 200 million users, will encourage many more developers to get involved.

It will also be seen as a major challenge to Facebook, the fast-growing network which opened up its site to outside software developers in May.

In a press conference in California, Google's chief executive Eric Schmidt and Chris De Wolfe, president of MySpace, said the two companies had been working on the deal for more than a year.

"The web has moved to its next stage," said Mr Schmidt. "We always knew the web would be social."

Google said it had "reached out to everyone" in the social networking industry, and indicated that included Facebook.

Real test

But the network, which was valued at $15bn (£7.2bn) after selling a 1% stake to Microsoft last month, appears to be sitting on the sidelines waiting to see whether OpenSocial takes off.

Tens of thousands of developers have now written applications for Facebook, ranging from online Scrabble, to movie reviews and video sharing services.

While Google has now staked its claim to be a big player in the social networking world, it is not yet clear just how big a threat it will pose to Facebook.

That will depend on whether developers decide it is better to work with a system which will cut the cost of producing new applications for all sorts of sites.

But the real test will be whether social network users care about just how open source their site is.