So far as I know, there is no standardized definition of meme, though they all share the concept that it's a piece of information that spreads.
The way you're talking about it, NW_Kohaku, it's acting more like a shibboleth (though I doubt the two are exclusive of each other).
There is a formal definition of meme, but it doesn't have too much to do with what people on the Internet mean when they talk about memes. It was coined by Richard Dawkins (the guy who's basically one of the most vehemently anti-religion speakers in the world), see Wikipedia:
http://en.wikipedia.org/wiki/Richard_Dawkins#MemeAnyway, yes, it's related in a sense to a shibboleth (which is mildly ironic, considering shibboleth is a word whose meaning derives from a Biblical passage), which is a defining characteristic of a certain culture - a meme, as Dawkins coined it, is a piece of a culture or sub-culture that gets imprinted upon people who partake in a culture, causing people to act in a similar manner. A meme is a part of a subculture (some concept or, in this case, catchphrases and images) that people who participate in the subculture have ingrained into themselves. Hence, "Magma is the solution to every problem" is something that people acquire as a piece of passed-on knowledge when they start visiting the boards. They learn that saying things like that are the accepted ways to fit in with the social clique.
The difference, for what difference there is, is based mainly on the fact that shibboleth refers to how outsiders of a group identify you as part of that group, whereas meme refers more to how the group indirectly passes on its concepts and beliefs to new members of the group.
Of course, if enough people mutate the term for long enough, it the language itself adapts to whatever arbitrary definition people have chosen to give it, anyway, so you could certainly say that there is
no longer a standardized definition of the term.