How can I know which friends on Google+ are not using it? I have reached the limit in some circles, and want to delete the users who are in my circles but don't use Google+.
I recommend the Circloscope Premium for Chrome: 5 AUD for the extension and your circles management will be a piece of cake.
Circloscope premium lets you analyze people in your circles in numerous ways. For example, you can:
I hope this helps, it sure has worked for me.