I have no idea about the origins of the word or the multiple meanings, but I first came across it in first or second year university when the professor used it in some social sciences class to describe something I'm going to have a hard time defining here. I guess, basically, the social uptake of an idea across a culture. The only example I remember (and I might be fucking this up), is basically the idea that (bear with me here) jeans are socially acceptable to wear to a bar, while sweatpants aren't (or something to that extent). There's no real utility-based purpose behind it, just the social perception that jeans are cool to wear in public, while sweatpants aren't. That idea was taken up by our culture and has basically become a part of it, and is thus a "meme".
Yeah...I probably got that wrong, but using it to define internet fads kind of makes sense in that context.
