Default privacy settings for your profile in Ning are set to, well...no privacy at all.  Anyone can view your page and all of it's content.  Here is how you change that if you want to (be sure to Display Images in this email to see the screen captures):

1. Go into My Page from the tabs at the top of the site
2. Under your name box in the upper right corner of the page find "Settings" and click on it 
3. Then in the upper left corner under My Settings find the second bullet point link labeled "Privacy". 
4. In there you can set all kinds of privacy settings, including only allowing your Page to be viewed by people on this network, or even just your friends on this network.