Linkin Park is "mainstream" by the very definition, they're on a major label, every time their albums drop, they enter no. 1 and radio play. However, though they are mainstream but, they're not lady gaga-bieber-kanye mainstream. You don't see them at every award show. The causal Listener might not consider them mainstream. Speaking of the casual listener, maybe that casual listener, listend to LP during the Hybrid Theory days but stop listening to them and nowadays when they come across a LP song, what reaction they might have? "I remember them." If a LP fan says "they need to go back to their old sound." Why would a causal listener say any different? However, at the same time Linkin Park has ties wit the underground. reffering to mike shinoda. He's worked wit styles of beyond, apathy and celph titled. (Who are rappers that I listen to) I guess what I'm saying is there are certain levels of mainstream.