San Francisco (Reuters): Shares of Twitter dropped about 1 percent on Thursday after co-founder Ev Williams said he is selling a minority of his shares in the social media company, which has been losing ground to Facebook and other fast-growing rivals.

"It actually pains me to be selling at this point, but this sale is all about personal context, not company context," Williams wrote in a blog post. Twitter in the fourth quarter posted the slowest revenue growth since it went public four years ago and its stock has dropped 16 percent in the past year.