According to the marginalia scholar H. J. Jackson, the golden age of marginalia lasted from roughly 1700 to 1820. The practice, back then, was surprisingly social — people would mark up books for one another as gifts, or give pointedly annotated novels to potential lovers. Old-school marginalia was — to put it into contemporary cultural terms — a kind of slow-motion, long-form Twitter, or a statusless, meaning-soaked Facebook, or an analog, object-based G-chat. (Nevermind: it was social, is my point.)
The author goes on to speculate and fantasize about how marginalia could similarly be shared in the electronic age. My reaction to his excitement over a twitter-like sharing of marginalia is one of overwhelm. How much can a person absorb? But read it for yourself and let me know what you think.