A lot of people are successfully "hitchhiking" on other people's ads.
For example: You see an ad pop up on your news feed from a competitor that is relevant to your own product or service. You then comment on that ad with a link to your own product/service and a mini sales copy promoting whatever it is you want to advertise. As your competitor's ad builds steam and starts to gain more exposure, so will you.
You're bound to get reported if you do this aggressively though so maybe consider buying a few cheap FB accounts to try it.