I don't want to be egotistical to say that I set trends, but I've noticed, oddly, that when I do or like something they start to become popular. In college I took a shine to green tea and then it became popular and there were suddenly like dozens of brands of it all over.
For a while in high school I always wore a button-up over-shirt and for some reason I would button just the middle button. Next thing I know, all of the popular kids were doing just that.
I've also listened to semi-obscure bands that became popular, though their popular albums usually sucked in their own right and not simply because they were popular (or were they popular because they sucked?). There's probably other examples I just didn't notice because I wasn't paying attention.
Ditching something once it's become popular is something I'm known, and made fun of, for with friends. Once green tea became all the rage, I said something like, "Great now I can't drink it anymore," and they pretty much knew I would say that, and of course reminded me that I was being stupid.
I joined Facebook when there was barely a million users, and I think had a black background instead of white, that was 2006. I don't use it as a much anymore now that everyone's on it.
I haven't used Facebook since 2004 and I'm shocked it became so popular.