You mentioned that it's not only happening online. I received a death threat in the streets of Toronto last week while I was walking home from work. How do we address this? I think part of the onus should be on these social media companies. They need to be monitoring in some way, because these threats are immediate. They need to be addressed in a timely manner, or violence will occur. It's that simple.
How to regulate these social media companies is difficult, because there are pros and cons. They do provide a great network to reach out to people like yourselves to find community where you otherwise wouldn't, but the threats of violence are real and very vast.