Shares of Twitter Inc. are down 2.5% in Thursday trading after the company said it has agreed to acquire Smyte, a company that deals with safety, spam and security. "Smyte's team, technology and company mission are aligned with our focus on improving the health of conversation on Twitter, and we believe this will be a powerful addition to our ongoing work," Twitter said in a blog post. The company expects the Smyte acquisition to help it "address challenges in safety, spam and security more quickly and effectively" through the review tools and processes that Smyte offers. Terms weren't disclosed. Twitter shares are up 153% over the past 12 months, while the S&P 500 has gained 13%.