[KongoSan]

I tried but it doesn't work.

First they will make claims about Islam which are utterly false.
Second they will not listen to any quote of Hadith, verse from Quran or anything you have to say to back yourself up.
Third, you will either get ridiculed by the forum members or asked to refrain from religious discussion at a certain point.
Fourth, there will be people claiming religion in itself and God himself does not exist entirely, let alone Islam.
Fifth, when you think you've had enough, they'll make blasphemous claims about the beloved Prophet Muhammad (p.b.u.h).

Dawah is best kept in person I believe. I've had nothing but bad experience trying to give dawah through an online medium.

Trying to give dawah online can really destroy your level of imaan and it came to the point where I almost started questioning myself. That is why I refrain from doing this online now. Since people are online and anonymous, they are more likely to make blasphemous and dirty claims. Face to face dawah is much more effective and people get to actually witness your character and humbleness as a muslim.