Simply answering a question that requires 2-6000 words of the Queen's English is not possible over a social networking site, what you often find with questions/assignments like that is that they often require an argument too, which can only really be created from the evidence you find in your research.
The way I see it, asking your peers for help is exactly the same whether you do it in person in a Library, or on a Social Networking site like Facebook, except when you do it on the internet you can do it from the comfort of your own home. And even then, the best you can get is ideas to go on for your essay, not a direct answer. In fact doing it can be a useful learning tool for seeing areas you have missed or should look at.
Granted, if it were a maths or science question, then yeah, you could get a direct answer over facebook, but again, this could be done in a library in person, so there really is no way to stop it, unless of course you ban coursework all together and just have exams (and even here, students can still discuss beforehand what they think might come up, ideas/hints/tips etc)...