Unlikely, the more people are involved, the more spam on the social networks and that is what they want, their ico will be for many people.
The purpose of bounty is to makerting and the more participants they promote success
I think this is possible. I see some campaigns from reputable bounty managers have limited campaign participants (social media). It is probably their goal to have the token that the participant of the campaign really has value. I think this is better than freeing the number of campaign participants if bounty is published by a reputable manager.