The real differences are in the radically different composition of an organism and an idea/meme/culture/institution, with implications for the way these ideas are transmitted and transformed.
As an evolutionary biologist and a student of cultural transmission this idea (memetics) has always annoyed me from both ends. It is a bad idea, and I wish it would die. It's even more annoying that it somehow manages to add to Dawkins' misplaced prestige.
isn't that because fads become "locked" in time? the fad is well evolved for the current culture but as the cluture changes the fad becomes very poorly suited for it, and dies.
i could make the same argument that you make against fads against any once-prolific-but-now-dead species, e.g. how could species x be wiped out by a change in its environment? that flies in the face of evolution creating a superior species.
evoltuionary advantage stems from a superior adaption to the current environment, and culture happens to change vastly more frequently than the environments most (if not all) species live in. hence the rapid rise and fall of memes.
I am not saying they are equivalent, just that the propagation of a fad has similarities to the propagation of alcohol-producing bacteria in a closed environment.
Why are we trying to make these ham-fisted analogies at all? Ideas do not behave anything like genes; they have almost no properties in common with genes.