You're forgetting the Like button. While people are more likely to follow people they're interested in on Twitter, on Facebook people are much, much more likely to Like things left and right.
This means that Facebook really has the best social graph. Not only is it often intimately interlinked through tagged photos, family relationships, work and education background etc, but you're also tagging things you Like as you go along in a fire-and-forget manner; something Twitter doesn't have. In fact, Twitter has very few of those things.