If the service is built to maximize the benefit to users (instead of to maximize benefit to advertisers, or to maximize a corporation's profits), then I'd argue that you cannot charge even some posters.
Maximizing the benefit to readers means getting the most and best content. Charging posters will eliminate some posts which is bad for readers.
And when a poster is willing to pay, that means the poster is profiting from the content, which means there's some motive other than writing the best content for the reader.
So my argument is that any financial gain for the operator, whether from advertising or directly charging users, will diminish the value of the service for both readers and posters.
If the goal is the best possible conversation, then let's look to non-commercial models that remove the distorting and corrupting influence of profit.
Maximizing the benefit to readers means getting the most and best content. Charging posters will eliminate some posts which is bad for readers.
And when a poster is willing to pay, that means the poster is profiting from the content, which means there's some motive other than writing the best content for the reader.
So my argument is that any financial gain for the operator, whether from advertising or directly charging users, will diminish the value of the service for both readers and posters.
If the goal is the best possible conversation, then let's look to non-commercial models that remove the distorting and corrupting influence of profit.