Let's not forget it happens in the actual
cities we live in as well! Hipster-ism gives way to gentrification and then the cool kids move because there are way too many yuppies. Same things happens with suburbs - they get too crowded and people start moving to the country to make new suburbs.
This is just human nature, it manifests itself in the clothes we wear, the cars/bikes we drive, the places we live, it certainly also applies to social websites. I'd be surprised if it doesn't - we'll see how facebook fares, or if there will be a coolkid exodus as the rest of the world gloms on.
It's basically a (sub)cultural arms race that everyone fights in.