I've already noticed that my Google+ feed is becoming dominated by posts along the lines of "Why am I still using this? FB is the same and all of my friends are there"

I expect them to have a fair bit of churn once the novelty wears off and all the invites stop hitting people's inbox

edit: I have a lot of non-techie friends not in the early-adopter crowd (who I invited) so I expect, for them, it feels pretty empty. For people who read HN, it's likely much more vibrant. Ultimately, it's the mainstream that matters and, until it hits critical mass, I believe the people we pull in from the mainstream won't see the point and will go back to FB where their news feed is always full.