It doesn't mean they're not active - although it does indicate they might be shy.
Your Google Profile (mine: http://www.google.com/profiles/louisgray) displays Public status updates to all visitors. It also will show you posts that were shared to a circle you were in. If you're in my Friends circle and I share to that circle, you'll see both Public posts and those sent to you... (Public + Friends)
So about these shy people?
While some of them may have signed up to Google+ and aren't yet ready to get started, or others are waiting for more friends to make the migration from older networks, there's a huge amount of more private sharing that you just don't see.
One of the best things about this network is the ability to selectively share, just like in real life, so if you don't know me, I never have to see anything you share with your real friends.
Maybe the real note should be "There Are No Posts Shared With You Yet". :)
Have shy friends? Tell them to share as much as they want with the people they want. It doesn't all have to be public.
/via My Google+ Profile.