There are plenty of things that are different among the services, but the fundamental difference is relationships and how they affect what people see.
Facebook: Friends are bi-directional relationships in which both sides have to agree to the relationship. In general, the things you post are viewable by all your friends, and show up in the News Feed if Facebook chooses to show it.
Twitter: Following is a unidirectional relationship in which only the follower needs to agree to the relationship (mostly). In general, the things you post are viewable by everyone, and show up on the Home page of your followers.
Google+: Circles are a unidirectional relationship in which the circle owner defines the relationship. The things you post publicly are viewable by everyone on the Internet, but show up on the Home page of people who put you in a circle. The things you post to one or more circles are viewable only by people in those circles, but again only show up on the Home page of people who put you in a circle. To further complicate things, you can add people to circles who are not Google+ users and opt to share things via email with them. Unlike Google+ users, they don’t have to opt in to get your posts.
So while Google+ superficially seems like a social network site akin to Facebook (it has profiles, friends, photos, sharing links/photos/whatever into the stream), the relationship model is much closer to Twitter’s. The question is whether real people will adopt Google+ with its targeted broadcasting model and full set of features over Twitter with its simple broadcasting model and lack of “clutter.”
All that said, it’s a little misguided to compare Google+ directly with Facebook or Twitter. A ubiquitous Google bar with notifications on Google, YouTube, Blogger, Gmail, Maps, News, Android, Chrome, etc. should go a long way toward increasing unique visitors and time spent across all Google properties.