FILE PHOTO: People holding mobile phones are silhouetted against a backdrop projected with the Twitter logo in this illustration picture taken September 27, 2013. REUTERS/Kacper Pempel/Illustration/File Photo

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.

Williams said in the blog post, addressed to shareholders and employees, that Twitter shares would continue to make up the vast majority of his assets for the foreseeable future.

Twitter, which last year failed in an effort to find a buyer, was down 1.14 percent at $14.36 in midday trading on the New York Stock Exchange.