I didn't have much of a social media presence at all when I submitted, though I had a GR and a personal FB. By the time I had my revision request, I'd made a Twitter and a blog. I also traveled down to Fantasycon to learn about the industry and also meet the publishers and show them I wasn't a crazy person (though perhaps travelling several hundred miles to meet them was a little crazy, in retrospect). After I signed, I made an author FB, got more active on Goodreads, and beefed up the blog.
Mainly, because they don't have a huge marketing budget, they want to know you're savvy enough to do at least a little bit of marketing of you as an author and your work.