please bear in mind that when yo advertiise on facebook you are targetting inactive customers who are not seeking your product or thinking about it at the moment you approach them.
Even if you select specific interests or behaviours when you target your audience on facebook, still these audience are not thinking about "buying" at the very same moment.
On the contrary, when you advertise on google using google adwords, you are targetting real active customers who are searching for teh product and in the process of purchasing.
For my business, I make use of facebook, intsagram and pinterest for marketing goals and building brand image and positioning, but for sales I use google.
Good luck :)
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