If Google+ fills a need for people, especially allowing people fine-grained, easy-to-use control of what is shared to who, then it should succeed.

Facebook is the big target that most people perceive, but surely LinkedIn is a huge target also, since Google+ circles should allow you to have work-oriented Sparks and pictures, versus personal-oriented status updates and pictures.

This shouldn't be a big deal for Twitter, since you might have many Twitter contacts where you simply don't know what their email is, but you still can read, message and retweet them, all within Twitter.