I specialize in a couple of niches and stick to those themes (natural health and metaphysical) I share a lot of posts on twitter/facebook. I also create Pinterest friendly graphics for all my hubs and pin them. I participate actively in Q/A and commenting on hubs and I also belong to a few different interest groups where I participate and am allowed to share info. NEVER spam - bad bad bad and it will backfire.
You can also cross promote. For example, I have several websites in my genres and I promote my sites via hubs and my hubs via my sites etc. It has brought me a lot of readership. Then of course there's the whole "SEO" business of optimizing your hubs and making them readable. I do this by thinking about what people are searching for when I write and then writing to answer a question or solve a problem. When people see value in your hubs they tend to follow and share your work.