Twitter, a communications tool for marketers, self-promoters and journalists, is trying to raise a third round of venture capital Silicon Alley Insider reports cofounders Evan Williams and Biz Stone are looking for $15 million, valuing the company at $60 million. Current investors include Union Square Ventures and Charles River Ventures. SAI guesses Stone and Williams have pitched Spark Capital, too. Twitter raised $5 million for a $20 million valuation last summer. The company still hasn't found a business model, mostly because users, by and large, view Twitter messages on cell phones or third-party desktop applications, not the Twitter.com website. We find this endless groping embarrassing. Twitter limits messages to 140 characters. Why not shorten it to 120 and slap "Sponsored by Coke" to the end of each message? That seems easier.