Google is once again challenging Facebook, this time by offering Internet developers an open system to create applications that can be used on various sites. 

The OpenSocial system is a set of standardised tools to help developers build applications and embed them in different sites – so there's no need for customisation or rewriting.
It is also a challenge to Facebook as it may poach some of the developers writing applications on Facebook to Google, since they could then re-use their applications.
Apparently about a dozen developers have already signed up.
Last week, Microsoft bought a share in Facebook, beating Google which had reportedly also been eyeing a stake in the social networking site.