Why Twitter's CEO turned down Facebook's $500 millionTwitter CEO Evan Williams always seems to be where the action is. He sold his blogging company to Google in 2003. He chased the short-lived podcasting craze with another startup. That company accidentally spawned Twitter, the microblogging service that's stolen the buzz from bloggers. It seems sensible that Williams would sell Twitter to Facebook, another social networking site that actually makes money. So why did he and his board turn down a $500 million offer from Facebook?Kara Swisher has typed up everything she knows on the deal. The short version: Facebook offered stock, Twitter wanted cash. Facebook wanted to use last year's ridiculous $15 billion valuation for its shares. The End. Williams always seems to know what's hot and what's not. Right now, his board is supporting his instincts: Build out Twitter's service before trying to make money from it, just as Google did for a few years. When the economy comes back, Twitter will be even bigger, and the offers even better — maybe even from Facebook.