"If you build it they will come" - that's only partly true. Ensuring you have regular content updates is important, otherwise there's no reason for anyone, once they have read one of your posts, to ever come back again. But until you get to the point where other people are sharing links to your content between themselves, you're going to have to do the donkey work of self-promotion if you want to increase your readership. It feels crass, it is crass, but it's what you have to do if you want to build up that readership relatively quickly. It can build up more naturally, but it will take a long, long, long time. Years.
Self-promotion can take on many different forms.
You can target tweets with relevant hashtags, even better if you put in the time building up a follower list (by following other people) of twitter folk who are more likely to like your content and spread it amongst their own followers. I'm not certain how Facebook works these days because I don't use it, but I expect there is some similar mechanic and thought process involved there.
Go to Wordpress blogs that have similar subject matter to yours, and start 'liking' and commenting on their posts. Then expand the net somewhat, and do the same for blogs that might not be on the same topic, but share something in common. Don't just do it once and think they will automatically start following you and sharing your content. This requires dedication. Then do the same for non-Wordpress blogs, tumblr, forums, whatever.
You want to be inserting yourself into a specific community of bloggers. There will be groups of people who regularly read and respond to each others work, and you want to become one of the crowd. You need to get your name known, so people recognise you when you comment. It can feel hollow and superficial, unless you actually start to enjoy being a part of this community, in which case everybody wins.
Everybody blogs. That's how it feels these days. Why on earth should you suddenly rise to the top and get a bunch of readers when there is so much white noise, an infinite number of monkeys typing away with their inane bullshit every day? Don't expect people to flock to you. And even if you start to get decent amounts of traffic, that doesn't mean it'll translate into comments or the sharing of your work. Think about how often you click on a link to something and then navigate away from it almost instantly. People will do that with your blog. All the time. It's the people who will comment and/or share your links you want to attract. After lots and lots of hard work, you'll likely find yourself with a small core group of people who will comment on your posts fairly often, in far fewer numbers than the actual amount of traffic you receive. That's normal. Hopefully some of them will start retweeting links to your blog posts, which will help bring in some more readers.
Blogging is more often than not a thankless task. It's a vanity project, and you have to learn to be okay with it being that so that you don't drive yourself crazy when you don't get asked to write for the Guardian after 2 months of diligent penmanship that you're certain everyone in their right mind should want to read. No one gives a shit what you've got to say about things, unless you give them a reason to give a shit. And that either means cornering a market where it hasn't already been cornered, or becoming a valued member of a community of like-minded people. Then, and only then, you might, possibly, perhaps, at a stretch, be able to forge a bit of a respected profile for yourself, and the traffic will - maybe - start coming on its own.
NB There's more in this very old post about building up an audience. I'm also storing up some ruminations about blogging after social media for the near future.