I don't know if I understand what true leftism is.
Tumblr leftism is, in my mind, basically a breed of college campus reactionary victimhood where you're supposed to be ashamed of yourself all the time unless you're black or transgender.
True leftism is just extreme leftism, either born out of a desire for leftist street cred and being emotionally adolescent.
By those definitions, I go with true leftism. I prefer that ideology simply because it's fits in with my beliefs in egalitarianism and constantly questioning authority. Tumblr leftism feels more like a cult to me.
I see it more like Lena Dunham vs. Dennis Kucinich.
I have no idea what Lena Dunham's politics are. Don't tumblr types hate her because her show is about white people?