i know im a little bit late in this conversation, but here is my take on the whole thing.
facebook, just like everything else in the world, is just a tool. like every other tool, it can be used for positive or negative. giving our children (and sometimes also adults) the information they need to properly use these tools is our job as parents and educators. many times (NOTE i did NOT say ALWAYS) this is a FACTOR in someones going off. teaching our children that it is forbidden may lead them to investigate later.
also, just a bit of nit picking. while its true that people have gone off the derech after using facebook, im almost positive that it was not the CAUSE. perhaps on the outside this is the way it appeared, but i would be willing to bet that there was some kind of vulnerability which was taken advantage of, even if accidentally.