Social networking company Facebook recently held acquisition talks with Twitter, the micro-blogging company. The negotiations put a valuation of as much as dollar 500 million on Twitter, which has become one of Silicon Valley’s most closely watched start-ups. Facebook offered to pay for the acquisition in stock. Putting a value on Twitter’s shares proved controversial. If it used the dollar 15 billion valuation at which Microsoft Corp bought a stake in Facebook last year, it would have valued the Twitter purchase at dollar 500 million, though that investment was seen as a high-water mark for Web 2.0.