Medium.com, the latest online publishing platform run by Blogger and Twitter co-founder Evan Williams, says it attracted 16 million global visitors in September, up from nine million in June. In addition, the two year old San Francisco company is making its first revenue from BMW in a six month deal which has the German luxury automobile maker sponsoring an online publication about design called Re:form, which launched on Medium in August. Both points underscore the fact that Williams, already…