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.