I don't know about instigram but on facebook you set a 'budget' for your ads and facebook will display your ad as they can within that budget. For example if you set a budget for your ad at $500 but it only runs certain areas totaling $300, you are only charged $300.
You can have facebook automatically take your money for that or you can have them bill you.
You will have to dig deeper to find out the value of your $300. For example if facebook tries to send your ad to me, I use strong ad-blockers, so I'll never see it. But I don't know if they still count that and charge you for the attempt.
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