To simplify things, only the number one "similar artist" in last.fm is used and displayed graphically as a vector. I know this is very simplifying, but if I used more than one vector per band, the nets would get too chaotic and harder to view.
The more listeners a band has, the larger it's name on the map.
That was it pretty much, so let's start...
...with the "core group" of folk metal, centered around the Finnish bands Korpiklaani, Finntroll and Ensiferum. While this branch is dominated by Scandinavian bands, it also home of the "Celtic" bands around Cruachan.
This is another larger group, consisting mostly of bands tagged as viking metal. Interestingly Thyrfing is the center, eventhough Falkenbach has more listeners.