Why Google Plus 'Failed'

Slashdot ran an article about Google engineers discussing why Google Plus failed. They point out that it was late to market, and that they wanted it to be too much like Facebook. As a developer, though, I am amazed by how well executed G+ is, but I’m also amazed by how closed the API is. I’m very surprised no one pointed out that the API is read-only. If I had to choose one thing that put me off the most, that would be it.