With Facebook you choose your friends/followers. With Twitter your followers choose you. With Google+ the friends you choose are mutually exclusive from the followers who choose you.
This means that every site has it's pros and cons and the content you would share with each is different.
I like facebook to keep in touch with my friends because all of them use facebook. There are also many more elements such as private messaging, event invites, and groups which work well in situations where you know everyone on your friends list.
I like twitter to keep in touch with people who like my writing on the internet. I don't have to know these people to give them information. To keep them following me I just have to keep my content interesting and consistent.
I haven't really figured out google+ yet so I just share awesome things I find on the internet and hope that the people following me care. I don't use it for much else.