After announcing the landmark agreement, and testing the service, Microsoft and Yahoo are going to be formally linked together on Wednesday. Although users have been able to sign up for a beta to access the new interlink, now that the agreement is formally finalized, all users will be able to mingle between the two networks with current versions of each instant messenger client. Although the service was originally planned for the second quarter of 2006, the beta didn't arrive until July 12 in 15 different markets.
The new interlink will create the largest combined instant messenger community at 350 million accounts, clearly putting the heat on AOL with only 100 million. Windows Live Messenger is available in 60 countries and is used by more than 240 million active accounts each month. Yahoo! Messenger is available in 19 localized versions with more than 110 million users.
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Update: Leah from the Windows Live Messenger product team talks about the marriage.