Yahoo! has decided to join Google’s OpenSocial .

OpenSocial is a common API (coding standard) for web site plug-ins that makes it possible for them to work on multiple websites.

The tremendous success of Facebook has made such standards increasingly important, as owners of social web sites would like to see as many applications as possible available for their users.

The idea is that the availability of such applications make the sites more sticky.

So far the majority of applications for Facebook have been quizzes and plug-ins fetching information from other web sites.

Google is trying to contain the success of Facebook by developing OpenSocial.

So far it has managed to bring sites like Engage.com, Friendster, hi5, Hyves, imeem, LinkedIn, MySpace, Ning, Oracle, Plaxo, Salesforce.com, Six Apart, Tianji, Viadeo, and XING on board. And now also Yahoo!.

Microsoft (who is trying hard to buy Yahoo!) is fighting OpenSocial as hard as it can.

It has now announced a new API that lets developers make use of the Windows Live Messenger contact list (i.e. the ID users of Windows Live services like Hotmail may have). John Richards, Director of Windows Live Platform, puts it this way:

Today I’m pleased to announce that Microsoft has partnered with some of the world’s top social networks on contact data portability. Starting today, we will be working with Facebook, Bebo, Hi5, Tagged and LinkedIn to exchange functionally-similar Contacts APIs, allowing us to create a safe, secure two-way street for users to move their relationships between our respective services. Along with these collaborations, Microsoft is introducing a new website at www.invite2messenger.net that people can visit to invite their friends from our partner social networks to join their Windows Live Messenger contact list.

This is not the same as OpenSocial, but is another example of how important your user ID is for the big online actors. They want your loyalty, even if it means sharing it with other sites and actors.