Memes are the most important cultural phenomena of this century, beneath the consideration of the mainstream until very recently. In the cesspools of the Internet, memes were cosa nostra, our thing. There was a pragmatic reason for their secrecy: when a meme is decontextualized by outsiders, the original meaning is removed and it becomes a simulacrum of its former self.
Memes, like genes, compete in an environment not unlike Darwinian natural selection. They must adapt in a manner similar to genetic mutation, and survive long enough on transient storage mediums to reproduce. In this regard, they are not unlike living organisms, in the realm of consciousness rather than matter.
The senescence of a meme can be defined by progressing from a faithful representation to a perversion, until the original meaning is lost. A meme dies when there is finality, an impasse in which new ideas can not emerge. This situation can occur when it is adopted for ideological means, by an unintended audience, or any other dead-end that forces a closure to creativity.
When a meme becomes mainstream, it has reached its terminal stage, as further evolution is no longer possible. By the time a meme widely recognized, its meaning has ossified and it's already dead. Memes evolve until they reach perfection, and then they die.