He was a member of a group named Buffalo Springfield. Oh, and he got pwned in a song by Lynyrd Skynyrd.
Neil Young was a member of Buffalo Springfield, but the heart of the group was Stephen Stills. Later Neil Young got added to Crosby, Stills, and Nash. Young had a solo hit with "Southern Man", which basically tarred all Southerners as KKK-grade racists. "Sweet Home Alabama" was Lynyrd Skynyrd's retort. Notice which of the two remains well known,

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Graham Nash was a member of The Hollies, and then of C, S, and N. After C, S, N, and Y, he kind of faded into an obscure corner of the music world, always protesting something and influencing few.
Joni Mitchell was big in the folk music scene, which was never very big in the US music scene. Her biggest hit, probably, was "Big Yellow Taxi", basically a Luddite protest song against modernity.
India Arie, I had to W'pedia. She seems to be almost big in the 21st Century Hip-Hop/R&B world.
Three never-grew-up Hippies and a 40-something almost-big hardly constitute an "exodus".