I believe there are a number of reasons why people have given up or don’t come on here anymore:
1) People’s interest in the club wanes when the team isn’t doing well
2) Fans expect Club Doncaster to provide the necessary investments to make the club successful again before they will come back
As Weighman said, there is a direct link between the declining number of fans and the number of people posting on the forum.
I suspect conflict between posters does stop some people from joining in. I’m not talking about differences of opinions, I’m talking about behaviour that crosses an acceptable line.
These types of contributions won’t encourage others, they will put them off.
It’s not all about being nicey-nicey but putting opposing points of view should be done with respect to those who don’t share them.
Perhaps, as S/O suggests, forums have had their day and people prefer to use other means of social media to say what they want to say.
We could really do with some of the people who don’t post anymore to give us a more definitive answer to the original question.