My impression is that 9gag is a social fad. Memes became mainstream and the 'cool kids' (the early adopters) are already making fun of le upboat XDD I mustache you a herp derp.
9gag is pretty popular in Israel[1]. My coworkers (20yo) still read it, but it was all the rage a few months ago, and in a family dinner my 13yo cousin asked me if I know the site. All this indicates the eternal cycle: 'knew them before it was cool' -> mainstream -> 'omg are you still into that?'
Dubstep (recent example) stopped being cool because it went mainstream. Memes will not stop being cool - image macros are the laziest way to get information - but 9gag (unless they do something crazy) will become just a site. Remember the esoteric tumblrs?