I already answered the first part your question at the end of my previous comment. As to the idea that they become more useful/beneficial than actual human members, I personally find actually interacting with other humans more fulfilling than interacting with a machine in a social context, regardless of how 'useful' they are.
If AI gets to the point where it's all pervasive, indistinguishable from real people, and we somehow cannot devise a way of weeding them out of online communities, I'd honestly probably just stop participating socially in public internet spaces, and instead restrict myself to private invite-only communities that do more intense vetting to ensure only humans are in that space, or participate in and help build up local mesh-networks, where people can actually meet up with who they talk to online.
That would probably mean a lot less time spent online overall, and instead going outside and finding connection with people in real life.