Well, I can't speak to the infectious model per se, but
   everything has its peak (and I think FB is at it or close to
   it). With no replacements, it'll continue but when someone
   'else' comes along, no matter how many ppl use it today, it'll
   be passe.

   Popularity. Comes and goes.

   Myspace had 300 million at its peak. Now FB estimates a billion.
   That's 3x as much. Surely enough to prevent demise?

   Personally, I don't think so. Numbers seem impressive but people
   are fickle and one false move with one good alternative and
   *poof* its population goes elsewhere en masse.