I don't think the authors would argue with you that a friends graph is more useful to real people. I think the theory is that an "interest graph" is more commercially valuable. If you follow Apple, you're very targetable. If you're friends with your college buddies, you are not.

I think Facebook realizes this-- which is why you're seeing them offering "like" buttons, share buttons, embeddable commenting functionality, etc. I honestly think Facebook is doing as good a job as Twitter at capturing interesting nowadays... And for a broader audience.