Rauf described communicating with Siddique Khan through e-mails, phone calls and, most usefully, Yahoo Messenger.

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Yahoo Messenger is one of the earliest online messaging tools, launched in 1998.

BBC: Yahoo to axe public-chat rooms feature

"We feel like IM is the communication medium of choice for the kid generation, " says Frazier Miller, director of product management for Yahoo Messenger.

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"I hate the idea, " says William Sanchez, who keeps in touch with his Californian colleagues 2000 miles away from his New York office via Yahoo Messenger.

BBC: Messaging in an instant

According to internet analysis firm Comscore, WLM still had more than double the number of Skype's instant messenger facility at the start of this year in the US, and was second only in popularity to Yahoo Messenger.

BBC: Microsoft ditches Windows Live Messenger for Skype

The software started life on Symbian and Series 40 devices before falling out of favor with Nokia, but this WP8 refresh enables cross-platform chatter between those older phones, the newer Lumia range and, as Yahoo Messenger contacts are supported, anything running that IM client, too.

ENGADGET

That means millions of AOL users can't chat online with the folks on Yahoo Instant Messenger or MSN Messenger, and vice versa.

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If AOL's services work together more fully, "it opens the door for us to argue for interoperability, " Brian Park, a senior instant messenger producer at Yahoo, said in Tuesday's edition of the Wall Street Journal.

CNN: AOL tests linking of instant messenger services

Variations on the Yahos malware have been infecting users for years, spreading initially via instant messenger platforms like AIM and Yahoo!

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Its Instant Messenger service is still inaccessible to similar services from Microsoft and Yahoo.

FORBES: Free For All