Where is the line then?

Isn't it obvious?

you are suggesting that someone posting something that is 100% false and known to be false is the line?

No, not at all.

I'm suggesting that someone posting something that is false and is known by him or her to be false, AND causes injury to another party crosses the line.

So, assuming your feet are of average size, I could say you have really big feet and put it on flyers and posters all around your neighbourhood. Nobody will care, and if any harm is done, it would probably be to my reputation for going to such lengths over something so obviously trivial.

If I did the same thing, only this time proclaiming you to be a violent sex offender and listing numerous crimes perpetrated by you, I would think the damage to your reputation and the potential bodily harm visited upon you by others in your neighbourhood, would be grounds for you to file suit.

In the case in question, if the posters comments caused harm to the company's reputation and to its stock price, and thus its shareholders, and the company believes it can provide evidence of such harm, it is absolutely within their rights to file suit and claim for damages. Posters on public forums are responsible for the content of their posts. To believe otherwise is naiive. It is not without precedent for action to be taken against them. If I recall correctly, during the dot-com boom, a young man was successfully prosecuted for posting false information (in this case, positive) for a stock he was pumping.

I hope that is clear.

For more information on the concept of defamation, see Wikipedia:

http://en.wikipedia.org/wiki/Defamation