These charts from Techcrunch tell the story.
In both traffic and socnets they host they have grown 5-6x in a year, and a quick bit of maths shows just how far they have travelled down the long tail. At 3.5m uniques in Feb and 200,000 socnets on the site, the average Ning socnet has 17.5 unique visitors. The maths is crude as it assumes all the visitors are only members of one Ning socnet and I would also expect that rather like blogs many of these socnets are idle, but my point stands even if the 17.5 is an order of magnitude out.
If the trend to niche socnets continues then data portability will start to become more and more important, which will in itself unleash another wave of innovation.