My gut feeling is that Ning paid access has not worked out as Ning expected it to. This is not Ning's fault. It is extremely hard to monetize a website. It's even harder to monetize a website by offering something that people will pay for, i.e. exclusive content. This is why we are seeing Glam trying to monetize through advertising. I've struggled to get people to pay through my pay wall, mainly because it's clunky, restricted to pay pal and in US dollars (not my currency). People simply ask for an invoice in pound sterling.
I have one group that is behind a pay wall. It contains about 30 articles. I have 45 people who subscribe to the service but only 18 of them did so online, ie paid using paid access to access the group. The rest insisted on an invoice and bank transfer for payment. Then, of course, the problem is that I couldn't comp them into the group so had to send them the articles as a PDF. Basically, it was all offline.