Someone above has mentioned facebook groups where it is allowed to post your referral link, from my own experience I know that this method doesn't work very well these groups have no rules, they are pure spamfests (about 5 new posts per minute) and almsot nobody will see your post.
But you can start your own FB group,set strict rules to prevent spam and pin your post on the top of the page,it's easy,free and you will get a lot of referrals.
In addition to this, you can also make your own FB page as opposed to a group, and put the link at the top of your page. They both are good methods, it depends on which one your prefer.
Also, there are clever ways to advertise sites, for example, I see people make tutorials for a certain type of site, and then put their ref link as one of the "trusted sites" to try out the method.