Enlarge / Medium CEO Evan Williams, as seen in 2010. (credit: Justin Sullivan / Getty Images News)

Medium, the San Francisco-based online publishing platform founded in 2012, has laid off 50 employees, or roughly one-third of its staff. The company will also close offices in New York and Washington, DC.

Ev Williams, Medium’s CEO, wrote in a lengthy post on Wednesday that the company would be changing its business model despite ending 2016 as “our best year yet.”

He blamed the entire concept of “ad-driven media on the Internet” as the root of the company’s shortcomings. As Williams, who is also a co-founder of Twitter, wrote: