The killer feature is that its blended with Gmail, and since a lot of people keep gmail open all the time it means you get notified and you see stuff. Not as common to keep one's facebook page open.
Do you have data on this? I'd think it would be close, if not the opposite.
Edit for clarification:
In my social circles, what you said is probably true, but I know many others use FB messages more than email (any email, not just gmail). Facebook has more pageviews total, and Facebook has probably around 3X as many active users (I couldn't find numbers at the same point in time, Gmail was at around 200 million last November as per the WSJ, Facebook was at 500 million last July and 750 million last week)
Don't get me wrong, I agree with you that G+ notifications in Gmail (and on Google Search!) is hugely powerful, and will mean G+ engagement among its users will stay quite high. But when you talk about "unseating" Facebook, you have to first come to grips with just how entrenched it is, compared to social networks that rose and fell before it. It is so entrenched that Gmail integration alone will not be enough - Facebook is substantially bigger than Gmail.
I've used Google Apps for work and since a lot of communication comes through email that means Gmail is open (or at least getting notifications with the talk gadget). I would not be surprised if you were correct that many folks leave Facebook open all the time.
I am obviously not sure this is a global trend, but I keep using the test "who did you receive an email from, recently, who isn't work or a fellow technical person?". Results are somewhat scary.
750 million people around the world use Facebook at least once a month. Of that number HALF return every single day, and 80% use it at least once a week. Facebook is a big deal that has deeply imbedded itself into the lives of many, many people. "Checking your Gmail" is a completely different kind of interaction.
At this point in time, I see no reason other than persistance of data (FB messages) to use the app.