Privately-held Facebook may never realize the notorious $15 billion valuation hammered out in its advertising-plus-equity deal with Microsoft last year, but at least that eye-popping number is good for something: Acquisition offers. The social network reportedly offered microblogging service Twitter $500 million of its own inflated stock, based on the same valuation metric as its deal with Microsoft, in a buyout offer that Twitter has summarily rejected.

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