There's a ton of dishy details on the social network that so many people love to hate, but ultimately I think it boils down to insight into two fundamental mistakes that Google made.
The first is a common pitfall for engineers: they fall in love with features and capabilities. They focused on building things like "social circles," privacy controls (ironically, as it turned out), and even wasted time arguing whether to enable logical operators in search, when the only thing that matters to a social network is the social element (namely, are the people you want to connect with already there)?
Technology is not the destination. Technology only matters so long as it's a means to an end -- in this case, a socially desirable end.
Google's second major error is similarly widespread: Google+ was always far more about what the company wanted than what its users wanted.
It was clear that Facebook, with its ever-expanding social graph, was developing an extraordinary dataset that could undermine the supremacy of Google’s key search product. At the time, Google had a relationship with Twitter to access the social network’s firehose of data, but that agreement expired in mid-2011. Google needed a way to get social data, and fast. ... These fears manifested themselves in what would eventually be called Google+. The vision for the product is clear, albeit complicated: create a social network for everyone that would simultaneously provide Google with enough data about each user to ensure its search engine could adapt to a more social world.Google needed user data, and so they created a network by which users would furnish them with data. The problem was that its would-be users, of course, cared quite a bit less about Google's data needs than Google did.
Whether you're building a product or crafting a message, the first requirement is to connect with the end-user/audience. Those people don't care what you want and need; they care about what they want and need. Focus on the former, and (like Google+) you'll end up with something that no one has much use for.