I think Google+ failed because Google doesn't care about humans. For Google humans are just a mean to an end. Google's implicit objective is to create an efficient computational intelligence.
It's not coincidence that Google, the king of the internet, overlooked probably the biggest benefit of the internet, improved human communication. The only successful product they have is gmail, just a copy of Hotmail with a couple of improvements. They tried the same with Google+, just a Facebook clone, but It didn't work because Facebook isn't interoperable as the email protocol.
I think It all comes down to the company's mission and to the objective of the leader. Some people think that mission statements and leadership aren't a big deal, I don't agree with that. The mission statement and the vision of the leader penetrate in the culture and the mind of the employees of the institution and have a big impact in every decision taken.
For example, Microsoft's mission was "a computer on every DESK in every HOME", so It was obvious that Microsoft was going to neglect mobile. In the mind of the thousands of Microsoft employees the computer had to be in a desk at home, anything beyond that was just a secondary complement. The web and mobile were accessories of the main computer experience at home. Microsoft was disrupted by the lack of vision of their employees, they were limited by a view of the world explicitly stated in the culture of the company.
Ballmer called himself a pattern, he was replaced with a very different leadership. Microsoft used to be seen as the evil empire. Nadella is described as a good man with good intentions.
Nadella first task was to change the motto, now It's "mobile first, cloud first", he will repeat that phrase a thousand times if necessary so the idea penetrates in the mind of the employees and influence their micro decision taking, their focus. Now Microsoft is the company investing more money in the cloud, and they're sacrificing Windows, their core asset, to get a foot in mobile with their services.
Google's motto was "don't be evil". But why not "be good" or "be generous"? It's really "don't be evil [because we need these people to fulfill our objectives]". People is a tool for Google, they use people to get money, resources and workers to build their projects. The projects aren't for humans, the projects are an end in itself, but humans has to be respected -while we're needed-.
The current Google's mission is "to organize the world?s information and make it universally accessible and useful", for whom? For people? Of course, because humans are the key economic factor right now, but little by little machines become a more important factor in production and the main consumer of Google's information.
Google is investing a lot in AI, they believe that real AI is possible. Humans are biological beings that evolve so slow, machines aren't limited by a biological structure, so the day machine intelligence starts anything is possible, they could do an equivalent of millions of years of biological evolution in just a couple of decades, or years. So, according to their investments, Google is organizing the information for that future artificial super intelligence, not for the messed up and limited humans.
Facebook's mission is "to give people the power to share and make the world more open and connected". Facebook is centered in people. For Google the most precious assets are the algorithms used to organize the information and the infrastructure of machines, for Facebook It's the billion of people in their social network organizing the information with Likes. Google's engine is a machine, and the ultimate beneficiary of all the activities of the institution is the soon to be super intelligent machine. For Facebook the engine is the human being, and the ultimate beneficiaries are human beings, machine intelligence is just a tool.
There are many differences in the views of the leaders of the tech companies. In an interview, Larry Page said that the most exiting thing in the last couple of years was a computer exploring the web and understanding the concept of a cat. One of the most interesting projects for him is driver-less cars. The name of mobile operating system he chose is "Android", compare that to iOS, or "Windows" (machines don't need windows). Gates is spending his time and fortune helping people, for Page charity is nonsense, he would donate his money to Elon Musk to develop crazy projects like the colonization of Mars, do humans really need to colonize Mars?. It seems both Page and Zuckerberg have problem interacting with people, but I think the difference is that Zuckerberg keep trying and Facebook is just the expression of his fascination for humans and their interaction. I think Page gave up on people, and Google is the reflection of that lack of humanity.
So, who will win?
Google's mission is to organize the information, but they're using the wrong tool, machines. I think real machine intelligence with consciousness is not even remotely possible, so machines will always be just a tool for humans, and machines won't understand humans and their context. Facebook dominates communications, Facebook is all the time listening to what people say and what people like. So, my prediction is that Facebook will disrupt Google using people as the engine to organized the information. The commercial key aspects of information is ads. Facebook will destroy the Google's business providing more relevant ads with its new ad network. In a couple of years Google will use Facebook ads in the search engine, youtube and their other products.
Google is doomed because Larry Page don't like humans, Google's implicit main objective is to develop an alternative to humans, and I think that's not possible, consciousness is the distinct aspect of beings and its beyond human comprehension.