Google+ already seems to be much more popular than Buzz.
How do you know? There are two reasons one may feel that a product is very popular:
1. You read about it all over the media
2. You hear from friends screaming at you to get on it cuz it's "sooo good!"
With Google+, it's almost all #1 and very little to none of #2. If anyone knows about facebook's early years, it was the complete opposite. The press barely knew about facebook(part of it was because you needed a .edu email to just get in). And yet facebook kept piling on users and even more importantly, engagement.
So sure Google+ may have convinced 40M of hundreds of millions of google.com users to click on a button and agree to opt in to google+, but that doesn't mean jack in the social network game. It's all about engagement. And Google+'s engagement is next to nothing.
My Google+ feed is significantly more active than my Facebook feed. I see hundreds of Google+ posts go by in a day, but only about 3 or 4 Facebook posts.
Google+ already seems to be much more popular than Buzz. I'm sure it will last quite a long time.