Yep, I'm calling it. Google+ is not a Facebook killer. You can all stop your arguments and forget about reading any other posts on it. The thing is - I don't think that's what Google is aiming for here. Twitter on the other hand, could definitely be in trouble. Let me explain....
I've only had access to G+ for a couple of days, but from what I've seen of how others are using it (and my initial instincts), the way it's being used is far more akin to Twitter than Facebook.
For me, Facebook is where I find follow the actual lives of people I know. Very few people share links on Facebook among my friends (granted, that's a sample of ~200 people, but still), most of the stuff is along the lines of "here's a loads of pics of my holiday", "I'm hungover", etc. Twitter on the other hand, is where I go to see what other people are reading and sharing. The number of personal comments is far lower.
G+ seems to fit much closer to this link sharing idea. What's more, the ability to focus who you are sharing with means that in theory the items appearing in my stream should be much more applicable to me. Just as an example using the sort of stories I share:
1. Tech news
3. Social Enterprise / Volunteering / Charity Sector
Those are three very, very different areas. At the moment, if you follow me on Twitter, you get everything. If you're only interested in one of those areas, you might get fed up with having to sort through the noise and find the applicable tweets. You unfollow me. With G+ that doesn't happen. I know who is interested in football, who is interested in tech, etc. So they go in those circles and voila! Now I can share more stories, because I don't think I'll annoy people who aren't interested in those things, plus the click-through rate should be higher for each link shared as it is more targeted. As Google tracks who is clicking on what, they build a social graph that is surely more valuable than knowing that X likes to look at Y's photos!
I think Google is happy for Facebook to become the social network for drunk party photos and inane updates about what people's rabbits did just now. They want to control the network for sharing stories and links....and that's why I don't think G+ will kill Facebook, however much people want to put the two in a head-to-head.