I think a lot of it came down to their "invite-only" launch. I remember a lot of my friends were excited to try it. Some of us were able to get invites, some weren't. Eventually the people who couldn't get on right away just said "screw it" and went back to Facebook. The rest of us followed because that's where we could interact with everyone and not just the people who happened to get on Google+. By the time it open…
Sounds like they were trying to emulate the early days of FB some people pine for where you had to have a .edu email address to sign up. I know a few friend were hopeful that Google+ meant they would no longer see posts from their racist uncle or gullible aunt. Start out with that same kind of exclusivity and hopefully it builds this exclusive mystique so when you open it up to everybody you get a flood. Perhaps the…
Fundamental problem: social networks with no-one on them are not fun and not sticky.
Fundamental problem 2: if anyone can read what you're writing, it's not a social network, it's a blog. So you can't just let anyone see what people are writing.
The way FB handled that in the early days was that by default, people in the same university network could see your status etc. I don't remember exactly what they could and couldn't do but the point is that they created a semi-privileged circle of people with some access based on a pre-made group that people would be familiar with - fellow students at the same university.
That gave people a network from day 1 in what felt like a controlled way. Also, as noted, they started with a desirable closed group that people wanted to belong to.