More and more people are self selecting into facebook and the advertisers follow. Google could fight it, but they would probably lose. There is an order of magnitude more people capable and willing to use fb and twitter compared to blogger.
Everyone was on blogs ten years ago. Had Google kept up their product, people wouldn't need to leave.
One could of course argue the value added by a typical facebook post, but let's not pretend that tens of millions of people were setting up blogs to keep their grandparents updated with pictures of their grandchildren.
Blogs are still here, I follow many of them and RSS is alive and kicking. Most people that had blogs before and produced long-form content are still there. The only problem is that the advertising well might go dry.