There are only 5 possible ad placements on Facebook!
None of these views is necessarily correct.
Tracking the customer’s journey to purchase is tricky, but Facebook claim to have the most accurate data. Last click attribution (it’s what you have by default if you don’t know what that is) only counts the last click before purchase. The “direct/none” and “keyword not provided” buckets often have social traffic, which is secure. If you look at view-through conversions on a 1 day, 7 day, or 28 basis, you can count the number of purchasers who saw a Facebook ad before eventually converting through a search result, email, or another marketing event.