How do you track ROI on your ad spend? The best way is to attribute it to the original source of advertising. You can get pretty granular.
If you use Adwords, here are some variables that you can pass through and track from your ads: https://support.google.com/adwords/answer/2375447?hl=en
If you use Bing, you can do something similar here:
You should never rely on your Facebook Ads or Google Ads Tracking.