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Skype has quietly added XMPP support to the most recent beta version of its Windows client, according to a report from Skype Journal. Skype for Windows 5.5, which was released last week, added Facebook integration to the VOIP service. A look under the covers reveals that this is done through XMPP, the open standards protocol that’s at the core of Google (s GOOG) Talk and supported by a number of other IM patforms.

Skype’s most recent Windows beta allows its users to have IM conversations with Facebook users as well as integration of Facebook contacts and news feeds. The folks at Skype Journal took a closer at the application’s network traffic, which revealed that it uses XMPP to communicate with Facebook’s servers.

This type of integration could in the future be used to achieve interoperability with other IM platforms. Microsoft’s own Windows Live Messenger currently doesn’t support XMPP, but companies like Yahoo (s YHOO) and AOL (s AOL) have been offering some support for the protocol. Facebook added XMPP support to its platform in 2009 to make its chat service more widely available as well.

By far the biggest supporter of XMPP is Google. Not only is the company’s IM service based on the protocol, but it’s also one of the main financial backers of the XMPP Standards Foundation. Just last week, the company announced that it was transitioning elements of its VOIP technology to Jingle, a signaling protocol based on XMPP.

Speaking of VOIP: The fact that Skype has started to support an open standards protocol doesn’t meant hat you’ll see voice or video chat interoperability any time soon. Skype has been using it own, proprietary encryption and communications protocols for its VOIP functionality, and it’s unlikely that Microsoft would abandon these technologies in favor of Google-backed open standards.