Racismovirtual is a non profit project in Brazil that aims to take the racist tweets offline and put them in the real world, in the neighbourhoods where they came from. If you have billboard media, you can donate it to the project. The project; ‘Virtual racism, real consequences‘ is run by a group of volunteers. The group seeks out bad tweets, finds out where it came from via the geolocation on twitter, and then gets a billboard space as near to the location as possible. By taking the comment offline and into the "real world" they hope to stir debate on how people react to these comments, and drive home the idea that even a tweet on the fly can hurt in real life.
The names and images of the people responsible for the tweet are blurred out, but to run into your tweet on a billboard in this fashion would be a jarring experience.
It's not all that easy to read the sender on the billboard unless you're a pedestrian, so I hope they're in walkable hoods.