From my perspective, the one thing Google got _really_ wrong with G+ was their APIs. When G+ was launched, tools like TweetDeck were heavily used for interacting with Twitter, Facebook and the like. All of a sudden along came a service that had no APIs by which you could post to it. Something you needed to specifically go and open a separate application for.

If they'd made public read & write APIs from the start, they could have picked up a massive initial user base as people used the tools they were already actively using. You've got to either:

1) Offer an amazingly compelling product with features that provide _significant_ reasons for people to compel people to use you

2) Go to where people are, and bring them to you.

G+ failed on both scores. It had good features, but they weren't _that_ compelling.