Interoperability

On October 13, 2005, Yahoo! and Microsoft announced plans to introduce interoperability between their two messengers, creating the second largest instant messaging user base worldwide: 40 percent of all users. The announcement came after years of third-party interoperability success (most notably, Trillian, Pidgin) and criticisms from Google that the major real time communications services were locking their networks.

Interoperability between Yahoo! and Windows Live Messenger was launched July 12, 2006. This allows Yahoo! and Windows Live Messenger users to chat to each other without the need to create an account on the other service, provided both contacts use the latest versions of the clients. However, if a user uses an older or third-party client, they will appear offline to the users on the other network.

Resource: Wikipedia