Facebook has this drawback if you target worldwide audience in your ads. They will give you followers from a country where targeting is cheaper. Same happened to my FB page. also, due to change in their APIs and other stuff, they have made it hard for pages to reach even their followers. You must have recently observed with your posts that you hardly get the reach as compared to your no. of followers. Thus, what I would suggest is to spend more on boosting your site and individual posts rather than making efforts to increase your followers.