Initially, it got a lot of buzz, people clamored to get invitations to join, and it became the fastest growing social network in history. But then... nothing.
And now, it's become fairly obvious that it simply hasn't found a place in people's lives.
The main reason given for lack of adoption is that people don't want to invest the effort needed to recreate the social network they've spent years building on Facebook.
That's true, but it's not the reason G+ did not take off.
"When’s the last time you got THAT frustrated with the Google search engine? Sure, you might not find what you wanted on occasion, but 99.99% of the time, it fulfills its function exceedingly well. So why on earth would anyone feel the need to switch to Bing? It may work yes, but to the average user, it doesn’t offer anything above and beyond what you’d find with Google, and in some avenues, is actually worse."
He makes a good point, but Google was not out to make just a marginally better product. Google knew it needed a fundamentally different value proposition, much like how the other sites have been able to carve out a community because they ARE doing something different.
Google thought it understood something important that Facebook did not. And it did. However, Google was still wrong.
Google's belief is that people's social networks are not, in real life, lumped into one single group of Friends. Facebook's Friends failed to reflect how relationships are actually organized. Google wanted to create a network that could elegantly accomodate different types of relationships and organize friends into independent groups, allowing users to send intimate updates to best friends and broadcast to the public with the same level of ease.
Paul Adams was working for Google when he gave the below presentation. It's a great talk, and it captures Google's thesis for why Google+ would work:
The result of this research was Google+ and its core feature, Circles. With Circles, you share each update only with the circles you select. One circle can have thousands of people, the other only three. It has the intimacy of Path as well as the broadcasting potential of Twitter (minus hashtags, etc).
Brilliant, right? Except that this isn't what people actually want. There are two unavoidable and insurmountable problems with Google+.
First, everyone who uses Google+ understands the rules of the game, and they know that they aren't receiving every update their connections send out. If there's very little activity in your Google+ feed, you could conclude that no one is using the service. Or, you could conclude that people are using the service, but are choosing not to include you in their updates. The latter assumption makes you feel left out and out of the loop, especially when you hear other people say in real life that they see lots of activity on their Google+. No one wants to feel left out and feel like a loser.
Second, people undeniably want to protect their privacy when they make updates. But, the over-sharing that is endemic to Facebook is a big part of what keeps users engaged. It feels like tabloid gossip when you notice that an acquaintance has taken down all pictures of their significant other after a break-up, or when you watch a heated argument break out in a comment string on a friend's wall post. In the manicured garden that is Google+, you will never be privy to this kind of content. You are left with only what you are already supposed to know about, and the news links that people broadcast to everyone. Nice, but boring.
What Google did not realize is that people don't care about posting privacy as much as they care about feeling like they are included and are in the loop.
If you give your users a great deal of flexibility and privacy when they are generating content, the unavoidable result is that users are left feeling uncertain about how much content they are getting to see, and what they might be missing out on. This is the crux of why activity on Google+ today is tepid at best and why, ten months after launch, the level of user engagement lags far behind Facebook, with no signs that the gap will close in the near future.