This is not a bug and it won't affect us if we decide to use Ning, but it something I did not intuitively expect.
If we set up the social network as "private" so that people need an invitation to join, then the invitation itself becomes a Friend request. So when Thomas joined this morning he made me a friend without further action on my part.
But if we "invite" all of the Facebook users, it's possible we will find that they have all become our "Friends" here. So we may have to be a little careful how we do that.