I believe that to be correct, Mark.
Thanks Otis. In that case my revised figure would be that $18,000,000 is possible in principle from only $2.50 a week each.
But as Justin quite rightly points out, most are not online. In the PR UK Advocacy group we're thinking about finding out by (a) getting a list togther of every organisation and support group in the country by asking people on foums to submit their local group's contact details. Then ideally (b) email each group/charity and ask them to quote approximately how many members they are able to get in contact with. It would be good to also ask under what circumstances they'd participate in mass rallying for campaigns. Anyway hopefully it would give us some basic statistical info about advocacy that's lacking....