Full Answer

The question of which source to trust presumes that one of the platforms has the correct number. None of them do — and this is by design, not by failure. GA4 reports conversions using last-click attribution within its own session framework. Facebook reports conversions using a multi-touch model that includes view-through credit. Google Ads reports conversions with its own lookback window and may include modelled conversions for privacy-restricted users. Each number is internally consistent but structurally incomparable to the others.

A first-party data warehouse resolves this by providing the ground truth: how many orders were actually placed, for how much revenue, by which customers. WooCommerce stores generate this data at the PHP hook level when an order transitions to a completed status. Server-side event pipelines capture this event and route it to BigQuery along with the transaction ID, order value, product details, and any attribution data that was captured at session start.

Once the ground truth is in the warehouse, each platform's claims can be evaluated against it. If Facebook reports 120 conversions for a campaign but BigQuery shows 95 orders with a Facebook-attributed first touch, the 25-conversion gap is likely view-through credit or duplicated events. This analysis is impossible without the warehouse anchor.

The practical discipline is simple: report revenue from the warehouse, report platform performance from the platforms, and never let one substitute for the other. Budget allocation decisions should be informed by warehouse-matched attribution, not by whichever platform's dashboard shows the best ROAS.