People will update on Facebook because they can ACCESS Facebook. I'm killing myself that I didn't get an invite when it opened up. I'd give my left testicle to use it. And as long as I'm locked out, I'm not going anywhere near Google+ and am forced to resort to Facebook. It's Google's own fault for pushing something out that wasn't ready for primetime.
I have one suggestion to Google... if they want people to migrate away from Facebook, make it so that your posts on Google+ get mirrored on Facebook with a tag at the bottom saying "via Google+". No sacrifice to users making the switch that way, at least in the interim.
Or even better, develop an algorithm that parses the data you download from your Facebook profile so that it can be added wholesale to Google+ so you're also not starting from scratch.
But I can't make these suggestions, cuz I can't access the frickin' service.