I am on a number of these .ning sites, and have received similair messages from most of them.
Unfortunately they have honed in on the 'open' type sites, and really make a pest of themselves.
I believe it's just a few people, and they prabably spend a lot of time 'marketing' their merchandise on these sites, probably getting a few hits too!
If someone finds a way to eliminate the problem, please let me into the secret - I regularly have to ban the offenders from rather peaceful social sites like http://www.cloudsnsunsets.ning.com and http://www.3putters.ning.com