Edit: I think I commented on the wrong post....
It is important to note that this demotivating effect is associated and likely caused by cognitive dissonance. I'm not in a good place to grab sources now but you'll find it on the Wikipedia page about cognitive dissonance.
The effect happens because we need an internal story to tell ourselves why we do something when we fail to get a reward. This story reinforces our intrinsic motivations, but when we are paid for the action we don't tell ourselves this story or stop telling it and so we get the opposite.
For this reason I suggest the lottery effect may make a big difference here. Whether you actually get paid for a post is actually fairly random/arbitrary. Most users won't get a meaningful payout most of the time. So, perhaps they will indeed be telling themselves the story of why they do this, and the effect you worry about will only happen for those who get a payout consistently or regularly enough that they don't have to deal with cognitive dissonance.