@bthall I tend to frame it more in terms of immediacy of the interaction + falloff. I think its close fit to your observation.
This always comes to mind on the topic:
http://headrush.typepad.com/creating_passionate_users/2007/03/is_twitter_too_.html
Another factor: Twitter, Facebook, Tumblr, and Instagram all seem to be subject to some form of algorithmic hiding too. Combines with tendency to prefer popular posts (gotta hit those engagement targets) & it further increases polarity -> sense of existence.