Douglas Rushkoff recently posted an excerpt about social currency from his forthcoming book. He first wrote about the idea a few years ago, and ran with it as the theme for this latest book.
A term like "social currency" can help explain a bigger idea like the meme, and Rushkoff's analogy about how we listen to the telling of jokes starts getting at the meat (which, I'm fairly certain, is lurking somewhere in that new book):
Observe yourself the next time you’re listening to a joke. You may start by listening to the joke for the humor - because you really want the belly laugh at the end. But chances are, a few sentences in, you will find yourself not only listening, but attempting to remember its whole sequence. You’ll do this tentatively at first, until you’ve decided whether or not it's really a good joke. And if it is, you'll commit the entire thing to memory - maybe even with a personalized variation, or a mental note to yourself to fix that racist part. This is because the joke is a gift - it's a form of social currency that you’ll be able to take with you to the next party.
This is a lot like an example of the two ways memes spread, which I seem to recall from Susan Blackmore's The Meme Machine. I don't have the book with me, so I'll go on as if this was in fact where I got this (it may have been Richard Dawkins' The Selfish Gene): memes are ideas - regardless of "good" or "bad" - that spread via imitation. Memes are the building-blocks of culture; culture is based on imitation. There are two basic ways we imitate, and you can think of them around this example of an apple pie: when you have a great piece of apple pie, you can either experiement with various ingredients in an attempt to arrive at the same pie by trial and error, or you can just get the recipe.
Imitate the result or imitate the recipe - these are the transactions made with social currency (I prefer to refer to it as cultural currency, but this is mere semantics).
In Rushkoff's terms, you listen to the joke for humor (the pie) at first, then attempt to remember its whole sequence (the recipe) -- so you can retell it (spread the meme, as it were). Consider for a moment the cheap imitations of the world, which attempt to copy the product without respect for the recipe. The recipe is worth more, culturally-speaking, because it does more to preserve the fidelity, fecundity and longevity of future results.