Matt Yglesias has something on memes today, and since they're one of our favorite subjects I responded on his comments writing this:

Yes, meme is a synonym for idea, but it emphasizes that you're analyzing the idea in a different way. You don't care about its content or truth value, but only its ability to spread.

Maybe its more natural for me to think about it this way as a mathematician, since we're often forgetting all additional structures and thinking about say the real numbers "as a group" or "as a Q-module" or "as a topological space" or "as a Hilbert space" etc. Memetics is just ignoring certain additional structures of ideas (like truth, or ability to make money, etc.)

Secondly, it seems the reason that memes are so popular among webloggers is that the internet, and the blogosphere in particular is a place where the ability of ideas to reproduce themselves quickly becomes most important. There are sooo many ideas floating around on the internet, and most of them just die in obscurity, but the lucky ones act like epidemics and suddenly you find that every website in the world is saying "Axis of Weasels."