There is no difference, under legal definitions - of posting a defamatory and slanderous attack on a person or company, on a website or forum - as there is in doing the same, in any other form of public dissemination - be it newspaper or magazine, or even word of mouth.
The laws of defamation and slander are quite clear and unambiguous. If you broadcast statements to the public, that are patently untrue, be prepared to be sued.
If you're expressing a robust opinion, you have the benefit of the law of "free speech".
Just ensure that your robust opinion is precisely that, and that it does not include malicious statements, and repeated, untruthful, assertions - that can be construed in a court of law, as seriously affecting the good public standing, and good public opinion of - or is to the financial detriment of - that person or company.
There are plenty of Rottweiler lawyers out there, prepared to make a name for themselves, by scoring lawsuits from financially well-heeled individuals and corporate bodies - who are prepared to spend substantial monies, to protect their good name and public image.