This is basic advertising. Google search ads and referral will bring in higher quality users because search is trigger/intent-based and referrals come from a more trustworthy source (usually), vs. FB ads that basically blast everyone. Even if you narrow down your audience on FB with interest targeting and proper demographic targeting, it may not help since many users just 'like' everything. Consider the supply and demand as well, the less competition on FB the cheaper your advertising is. Perhaps competitor also realized FB was the wrong way to go and stopped advertising on FB, that means more auctions for you to win and cheaper users. Of course, that doesn't mean higher quality users.