Why Google+ is the place for passions

And that's exactly right.

So if Google+ is a social network for people to share their interests, what are Twitter and Facebook

In general, Twitter is dominated by news, celebrities, pundits, professionals and narcissists.

Facebook is mostly about family and friends. And narcissists.

And each social network draws people who are seeking the type of engagement a particular network specializes in. That's why Twitter and Facebook people don't get Google+.