It isn't particularly useful to say nothing more than "you're wrong" without explaining why, although sometimes a post tries to make a point but the reasoning is flawed such that it doesn't actually demonstrate the point it claims to demonstrate. In a case like that it can be difficult, and maybe not really relevant, to make a counterpoint because the counter to the post is to say "your post doesn't prove what you say it proves" - if the post doesn't actually prove anything at all there is no counterpoint to make beyond that the reasoning is faulty.
To take a simplistic example, if someone posts "the sidewalk is wet, therefore it is raining" it's perfectly fair to point out that the logic does not follow. The fact the sidewalk is wet might mean it is raining, it might mean that it has been raining, it might mean the neighborhood kids had a big water fight, it might mean someone washed their car, it might mean all sorts of things. We can agree the sidewalk is wet but where the poster drew a faulty conclusion from the observation the reality is that we cannot draw any specific conclusion. It may even be that it is currently raining, but that conclusion cannot be drawn from merely observing the state of the sidewalk.
