Four years ago today I started @Twitter. It was the fall of 2009. The company had just closed a new round of funding in September. Lists were introduced to the service that October; native Retweets were added in November. As of December 7, there were ~100 employees. My first day happened to coincide with the company’s first day in its new building on Folsom Street. There was an incredible amount of energy and enthusiasm in the office, but with no clear business model and serious site stability issues the future of Twitter was far from certain at the time.