I’m often asked if a social media / online platform matters.
The short answer is. No.
The longer answer is that for fiction, the most important thing that publishers care about is the quality of the writing. If your twitter following / facebook following isn’t significant, it is not worth highlighting in your cover letter.
For non-fiction yes, a social media / online platform is relevant. If you’re positioning yourself as an expert in a field, it is much more convincing coming from someone with an established following.
If you are a novelist, focus on the novel, not on the social media following. I only google a writer’s name if I’ve called in the full manuscript, and I’m curious about a writer’s background, but if there is no online footprint, that isn’t detrimental (I would probably think, hey, this person is writing instead of using twitter – nice.)