They missed out on not including Reddit in the social networks. Reddit seemed like meme-central this election for Sanders and Trump.
A good portion of reddit's memes are hosted on imgur, which was included in the dataset, so there's that.
Too many CTR shills on reddit
How do you distinguish between legitimate Hillary supporters, and CTR shills?
And, all of the underlying data is available on Kaggle: https://www.kaggle.com/SIZZLE/2016electionmemes
Here's the working paper: http://iandennismiller.github.io/election-memes
EDIT: quick link to viz: http://imgur.com/S6nHNLT