+Jeff Jarvis , I think you're close. Here's my take from inside and at the edge of higher education.
* Facebook is for relationships
* Twitter is for broadcasting
* Blogs are for writing
First, let's agree that we don't know what G+ is. But initial signs strongly suggest that G+ is moving into the 'relationship' and 'broadcasting' spaces, and it is doing so with the new value-add of taking FB's vanilla social graph and allowing its owner to categorize (in a one-to-many way, even) members according to how they are related to the owner. G+ brings contexts to the table. So chances are, G+ could turn out to be a tool for sharing information in context-sensitive ways. It could also be for broadcasting in context. So G+ might be moving into the FB and Tweet spaces.
Currently, people are finding ways to force the round pegs of FB and Twitter to solve the square-hole problems of context-specific communication. (The NYTimes On-Line had a story about this phenomena not long ago; alas, I can't find a link to it.) I have two FB accounts, one for my family and personal friends and the other for professional relationships which include students. I also have two or three FB Groups, one for each of the NSF-supported projects that I've run in the last five years. It's tough to maintain and contribute to so many separate facets of my online presence, so the secondary outlets (e.g., the FB Groups) mostly sit unused. Which is a pity, because their audience is interesting and important to me. Now, with G+ and its baked-in Circles, we may be seeing a solution to this problem.
So, G+ may not change the social networking landscape in a way that shifts our FB/Twitter/Blog paradigm. But G+ may change the way we use these tools. All users with nontrivial social networks and players who provide networking tools should be intrigued by that.