In the rational model, members keep participating until the costs (the time needed to participate) outweighs the benefits (good information).
In practice, this isn't the case. At least not in the common understanding of costs and benefits.
The costs include social risks and opportunity costs. Would members get more value from television? Is participating in the community emotionally expensive? Is there a fear from participating in the community? Or a fear from missing out? Will other people critise me for something I've said?
The benefits include social value. Does being part of this group give me a stronger group identity? Do I get higher levels of self-validation, efficacy? Am I making friends with people in this group? Is this is a group where I can be myself?
We need to tweak the formula away from just time and information, and towards the range of other factors that influence the level of participation in communities. Simply by lowering the risks of participating, increasing the efficacy of participants, for example, you can significantly increase the levels of participation.