Remember Hillary's great social media strategy?
It works better when you have someone who can actually appeal to young voters without it being cringeworthy. See: Obama 2008 & Obama 2012.
So someone who isn't Cory Booker?
I understand dislike Booker's politics, but what makes his appeal to young voters cringeworthy? Personally, I think he's very effective at that and has a great opportunity to draw large millennial support based on his rhetorical style and ability to connect.
I think he has a very strong "How do you do, fellow teens?" vibe to him.
I feel like the Dems' Twitter account is like that. I just feel disappointed whenever I look at it. So many memes and gifs.