I agree with you about the arc of social networks but I feel that the case of Google+ is a little different. Though more difficult to find, I think the organic conversations continued and may continue until the lights go off. I think that G+ ultimately failed because of lack of a cohesive - and simple - vision of what it should be.
I don't want to move this conversation to the fate of G+ (there are other posts for that) but the cautionary tale of G+ is you need to two ingredients to make an interest-based network successful: Vision and love. That's the burden for Cake to deal with now.