When the person is having a rant, they are trying to be constructive (“positive” has the same meaning as in “positive criticism”). The other is as an intensifier.
I used to rant in Facebook. I still want to rant because of too much stress. I chose to be quiet but seems that there are a lot of triggers.
Go and rant. When people seems to make stories turn upside down and you have dealt too much. Sometimes, they want attention and manipulators are angry when they see that you are too strong to handle stress.