Microsoft and Yahoo have confirmed they will allow their instant messaging users to talk to each other by integrating the infrastructure used to support the systems.
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The combined instant messaging customer base of the two companies will be 275m and the move is seen as a challenge to AOL, which still operates its own proprietary IM infrastructure but which has the most IM users.
Interoperability will start during the second quarter of next year, the two companies said.
The deal will allow MSN Messenger and Yahoo Messenger users to exchange text messages and contact lists, and carry out PC-to-PC voice over IP chats.
As well as further challenging AOL in the consumer portal advertising market, the alliance is seen as an attempt to stave off the threat of Google, which is rapidly expanding its desktop and communications services via its portal.
MSN Messenger, Yahoo Messenger and AOL’s AIM system can already be integrated for business use using Microsoft's Live Communications Server 2005.