The Facebook messaging uniqueness stems from the fact that such messages can be received in any medium or device that is convenient for the receiver.
It offers seamless messaging across different forms of communication mediums: in addition to aggregated conversation history between two Facebook users, and what seems like a copy of Gmail's 'Priority in-box' termed 'Social in-box'. The 'Social in-box' weighs messages based on user priority alongside conversation history and will help in filtering out spam.
According to an official blog post, the actual service launch will follow gradually along with making an email address available to everyone over the next few months.
Indeed, social messaging capability holds the key to the next generation messaging system, as attested by Google's earlier foot print on that domain with its Buzz service. Also, implicit as it where, all communication will happen within a single domain, but as to the extent of success the Facebook trip will go, only time will tell.
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