I have no idea how old you are, if you've ever really used web forums, but they can be quite robust. This is obviously, clearly the technology to use. I don't understand why you think there's some limit, some ceiling on the number of users it can handle. The only limits are bandwidth and storage space... And with 70 million activists (as if everyone would be active on the forums, but whatever) with that many people, there will be donation funds for supplying the necessary bandwidth.
There's just no problem here.