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.