I see a hierarchy of ephemerality with Twitter at the most vacuous, FB next, web forums, NNTP and then good old fashioned email lists

I suspect one reason for the popularity of FB is that there is a dedicated app for smart phones and tablets that handles images well. Reading a forum on a small screen device can be a pain unless a decent Mobile HTML5 version has been produced.
Twitter has its place perhaps for a particular event when rapid dissemination of messages is really required within a group on the move as it were.