In Facebook's case, I think they are already serving completely different content to logged-in users and non-logged-in users/crawlers; most of the complexity of Facebook is invisible to crawlers anyway.
To me it sounds that BigPipe is as applicable as Flash or GWT - solutions already known for their problems with SEO.
However, if that's not a problem for you then it's probably a good optimization for your site.
Keeping two different output formats for a site (one for crawlers and one for humans) sounds complex. To date, none of the sites I've developed could have justified such overhead in development.
Keeping two different output formats for a site (one for crawlers and one for humans) sounds complex. To date, none of the sites I've developed could have justified such overhead in development.