I often heard this so-called opinion lately (which often occurs in facebook), that when someone says 'i don't want to be mainstream, i am anti-mainstream', then someone else would comment 'you know, anti-mainstream has become the new mainstream', and the first guy would then reply 'oh yeah you're right' and nod in agreement, and everybody seems pleased.
I've heard it one too many times that I've been wondering myself and I came to a conclusion that it is wrong. Why is it wrong? It is simply because there could only be one state of being mainstream--at a time. And when the mainstream becomes minor, the anti-mainstream becomes the new mainstream and the other way around. It is so fluid and balanced and the term is already logical and well-defined.
So, when you say you're anti-mainstream, you are anti mainstream no matter what, rejecting whatever comes as mainstream at that point of time, with no connection to past or future condition. So unless the shift of what is mainstream at any point of time to the other point of time is way too rapid to follow, there should be no problem of defining what is mainstream and what is anti-mainstream.