Who is responsible for content posted by a user on the Web? The site owner or users themselves?
For example, who is guilty if one person insults somebody else using his blog or talk-back? The person or the site where he writes his texts?
I believe these details are always outlined in the legal documents. Terms of Service, and other policies... Most countries uphold freedom of speech however this may not hold true if any domestic or international laws are being broken.
Whenever in doubt regarding legalities, it is always best to consult a lawyer.
In practical terms (from a business, not necessarily a legal perspective), you should assume that you are responsible for whatever content your users post. Your hosting company, and (as unlikely as it seems) even your domain registrar reserve the right to terminate your service if the actions of your users infringe on their TOS.
This will vary from legal system to legal system. There is no global answer.
A good lawyer, from a large firm used to dealing with these issues can at least give you practical advice based on your situation. You haven't given nearly enough information for anyone to advise on, and it wouldn't be worth you doing so anyway.