I create a new thread myself every other day or so with some CTA type title, tweet about it with a format of %title% - %url #somehashtag #somehashtag, then go to ads.twitter.com and promote that tweet using a twitter follower campaign with a 60 cent bid
My ad campaign gains me about 10-20 twitter followers per day, plus my wordpress content gets me another 10-50 (wildly variable) per day
My facebook page, I post all of my wordpress articles to, and pay 1$ a day for facebook likes
The facebook targeting and twitter targeting are primarily focused around hitting people who have already seen my site, but do not like or follow the social accounts.
This method of targeting causes significant engagement, as they already know your brand.
All in all, spend about 150$ a month on advertising on a site that pulls in about a grand per month (though the forum itself is not monetized at all, it's to raise community engagement with the main wordpress)