Have you got your Google Analytics all set up correctly? You should be able to compare the source of your sessions (the visits people make to your site) vs. the source of your orders.
It would be worth ensuring that your Google Analytics is set up with the correct filters too, to remove spam traffic made by robots, which can skew your results. The vast majority of sites we look at don't have filters set up and as a result it makes their conversion rate look worse than it actually is.
If you need more specific advice or want me to take a look into something for you I'd be happy to. Feel free to drop me a line via email.
Stephanie here from Shopify's guru team.
I would also recommend utilizing Google Analytics to track your store's conversions. If you've yet to set it up, we have a helpful guide here - https://help.shopify.com/manual/reports-and-analytics/google-analytics/google-analytics-setup#turnin...
Also, you can utilize the Facebook tracking pixel to retarget, optimize, and track the effectiveness of your advertising campaigns. Of course, we also have a guide to help you with using the Facebook tracking pixel on your store - https://help.shopify.com/manual/reports-and-analytics/facebook-pixel/getting-started
Hope this helps!