negative feedback

In these days of social networks, recommendations and feedback, how do you give seriously negative feedback?

I would like the world to know how useless a former staff member was, but can’t bring myself to write comments directly on their linkedin profile (christ, we were just glad to get rid of them!), but almost every day at the moment, I am picking up the pieces of this person’s appallingly unprofessional work and cursing that my employer was somehow tricked into paying them twice as much as they pay me! Pity the next poor bastard that has to work with them.