I think it's possible to change the minds of others through online posting. It has happened to me and I think I've effected change in others. However, all online posts are not equal.

Posts like GP I think are likely to have the opposite effect. Bad faith argument tends to let people dismiss any valid content and reinforce existing views and groups.

I find the key is to be respectful and to make an effort to understand where the other person is coming from. This unfornately appears to be a less and less common tactic.