I look at everyone around me, and they would be open to leftism except for two things: 1) They only believe whatever is shared within their group of friends, liberal ideology makes them disregard anything else as "crazy" and 2) they don't have long-term coherent trains of thought, it's just a constant stream of sensation, one moment they're looking at a meme making fun of "anuddah shoah" and the next watching a John Oliver clip and the next listening to Rihanna.
Maybe this would happen via other mechanisms if it weren't for social media, but social media is how it happens today. What we need is an affinity group structure to infiltrate groups of friends and politicize them. That is, we need people who agree on common ideas to agree "I'll become friends with this group of people, you'll become friends with this other group", etc.
I was a liberal only a few years ago, I know how they think. They think of themselves as rational, but in actuality make subconscious appeals to authority all the time to justify their own thoughts. Just arguing with them straight up doesn't do a thing - they'll just ignore you or, worse yet, say that you're a Stalin supporter or something like that, and go right back to watching Samantha Bee say that the Left and the Right are the same.
Moreover, if there ever is a problem which they can't ignore, the first place they look to is the "approved resistance", which is of course just more extreme idpol. This is how everyone hates SJWs and yet they just keep popping up. We need to position ourselves as the alternative. If it means someday doing something violent to an SJW and doing so clearly as a socialist or anarchist, so be it. We want them to call us manarchists and brocialists, to make themselves look dumber than they already do, to have people realize that the divide between liberals and the left is much bigger than the typical sectarian infighting between Marxists and anarchists.
In conclusion, I think millennials have potential to be awaken, in fact we're just criminally bad at doing it. It's right there. You just need to break the social bubble. Make them question all the work they're doing and why. Call out idpol with memes, because rationally arguing against it rarely works. Perhaps most importantly, connect what they already know (1984, Einstein, etc.) to socialism.