Here's the breakdown for me by social network:
I use these separate social networking sites to reflect my different social circles. In particular, unlike many people, I've strongly resisted the urge to add friends willy nilly to Facebook. People with 500 friends on Facebook are kidding themselves if they actually think that many people care about what they are doing. That said, I use Twitter for sharing with everyone, but at that scale it's typically more of a broadcast medium than a social medium, and I use it accordingly. Of course, Facebook lets you share publicly as well, but then conflates all posts into a single unmanageable stream. Finally, LinkedIn, is basically a rolodex in the cloud, but still useful for keeping track of acquaintances and colleagues.
Why have 3 social networking sites when you can have one? In the real world, we effortlessly communicate different messages to different social circles. In other words, we share information selectively. With Google+ you can now do the same thing when you're online. You can selectively share to social circles, and then keep those conversations confined to their respective circles. Family stuff for family, friend stuff for friends, work stuff for colleagues, etc., you get it.
I've just touched on one exciting feature in Google+. If you'd like to learn more about Google+, read the introductory blog post.
One last thing. If you follow me on twitter, and would like to try Google+, sign up here. I make absolutely no guarantee that you'll actually receive an invitation though.
And remember to +1 this blog.