I never used Diaspora so I can't comment on that.
Your point about seeing the same content three times or whatever is invalid because it assumes that people use FB, G+, and Twitter in the same manner. That's not the case. Each has it's own "personality" and people use them in drastically different ways. As do I. I have fans and friends who follow me on Twitter, are friends with me on FB, and have me circled on G+. I don't post the same things on all three. Actually I barely use my FB and I feed in posts from Twitter for the people that don't already follow me there. But even people who follow me on Twitter might want to friend me on FB because of people tagging me in pics, videos, etc.
On all three, it's a mix of friends, fans, celebrities, family. All kinds. When I said G+ was more engaging, I mean that (for example) if someone does post the same thing on FB and G+, it seems that it starts a better conversation on G+. Rather than people just clicking "like" and making a short form comment which seems to be the norm on FB - on G+, there's more value in conversation, which in that sense is similar to Twitter but without the character limit.
Instant Upload also makes it more convenient to share pics. I don't remember the last time I uploaded a picture to FB (on my personal profile, obviously I have to keep the music stuff updated).
The Circles feature is still one of the best things. I have a circle of friends interested in pro audio, for example. I know damn well that nobody else I know outside of that circle gives a fuck about how I discovered an awesome new reverb plugin or how to use parallel compression so your drums don't sound weak or the latest mastering techniques. So I'm not gonna post that stuff on Twitter or FB, but I can post it to that one circle and start a great debate and conversation that nobody else who doesn't care gets annoyed by.
Also - the UI is beautiful. Using facebook just looks like some caveman shit to me now. G+ has a much better interface.
Maybe it'll be the case that the minority of "power users" create most of the content. But that's fine, because Twitter is the same story. But when it comes to content sharing, I seem to be clicking that +1 button on various sites more and more, and that "Like" button less and less. Because not every story or link is suitable for everyone. In fact, most of them aren't. And as G+ gets integrated into more products, like Google Music, Google TV, Google Reader (I think this is about to get rolled out), etc etc etc, it just makes the whole process much more painless. Technologically, G+ is FAR more advanced than FB. Go edit a photo on G+ using the new built in editor and you'll see what I mean. That and the UI are just two examples. I'm not expecting everyone to stop using FB and start using G+ overnight and I never said otherwise. BUT - as that technological gap increases (which it will), more and more will make the switch. Which is exactly the same as what happened with MySpace. Also, spam. Every day I have to delete tons of spam from my FB inbox. Flawed system. Turns people off. Again, this happened with MySpace. I've yet to see any spam with G+.
Side note, I showed a bunch of my cousins, nephews and nieces the new FB "Timelines" layout, which most people don't yet have, but will be rolled out to everyone, like it or not, within a year. They all hated it. Which is funny, because I actually prefer it to the old one and I found myself being slightly more active on FB once I enabled it through the developer program. But when it comes to "the casual users", people are going to fucking HATE such a drastic change.
Anyway, it's not a zero sum game. G+ is a better, newer, more 21st century system. FB is 8 years old and bloated to fuck. But the user base is huge. I don't think everyone has to leave or stop using FB for G+ to get popular. But when I hear people saying G+ usage has dropped off, or whatever, I have to scratch my head in confusion. My stream on G+ is more active than it's ever been, I get circled by a bunch of new people every day, and I find myself sharing more and more to it, simply because I prefer the results and it's more convenient.
I see it becoming even more convenient as they build in new features, and from ICS onwards G+ is weaved into the very fabric of Android, which itself is showing no signs of slowing down (It's been suggested that it'll reach 1million new activation EVERY DAY in the not too distant future)....
and there's one mega card left to play. YouTube. MOST content shared to Facebook is what? YouTube videos. But you have to do it manually, right? Copy and paste that link. Have you seen how simple it is to do the same thing to G+? It's one click. And better believe YouTube ain't gonna have a "Like" button any-motherfucking-time soon. With the new $100 million boost Google just put into over 100 channels of original content on YouTube (including a hip-hop channel by Jay-Z, a Hispanic channel by Sofia Vergara, a dance channel by Madonna, a comedy channel by Ashton Kutcher and much much more), plus the planned integration of Google TV 2.0 with large numbers of forthcoming Sony, LG and Samsung TV's..... well, I think it's pretty obvious where the potential is.
It boils down to convenience because people are lazy. I never said G+ would come out and obliterate FB within months of launch. Google never thought that either. It's a long term slow burner. But for as many users as FB has, the reason Zuckerberg is shitting himself is because Google owns all the keys to the internet, when you really stop to think about it. What would be worse for the internet - if Facebook disappeared tomorrow? Or if Google.com, YouTube, Maps, Gmail, Chrome, Google Docs and GCal went?
It's like internet monopoly. Facebook has built a huge fuckoff mansion, but Google has properties on every side of the board. When you roll that dice and move your game piece, who's in the better position, the one square that if you land on it, you pay a huge sum, or the other player with a whole bunch of vital properties that you can't help but land on?
......I wish my brain wasn't so freakin active at 2am when I've been up and working all day.