Use of the Facebook Conversions API is growing as technical issues increasingly prevent browser based pixel events from making it to Facebook. The responsibility of advertisers to justify ad spend remains even in such scenarios, and the Conversions API bridges the growing gap from missing pixel based events in measuring return on ad spend (ROAS ).
Where an increasing proportion of pixel events are failing to be delivered, Facebook standard and custom events can be sent from both a pixel in the browser and also from the server via the Conversions API. In this instance, an advertiser must take care to deduplicate pixel and server events to ensure accurate attribution reporting.
Pixel and server event deduplication
If Facebook finds a server event with parameters event_id & event_name that match a browser event with corresponding parameters eventID and event sent to the same pixel ID within 48 hours, Facebook discards the subsequent events.
02/25/2021 update: You can now use event_name, Browser ID (fbp) and/or External ID (external_id) consistently across browser and server events for deduplication as well.
Note that in this scenario events are only deduplicated if they are received within 48 hours of when Facebook receives the first event with matching event_id / eventID parameters. If a server and browser event arrive at approximately the same time (within 5 minutes of each other), Facebook favors the browser event.
Event merging
Merging considers browser & server events that are candidates for deduplication, and if they have arrived within 10 minutes of each other, event parameters are merged. This means for two events that are candidates for deduplication, for example, if one event has a value for parameter A but not parameter B, and the other event has a value for parameter B but not parameter A, the merged event will be assigned both parameter A & B values. Events that arrive more than 10 minutes after each other are not merged.
The more complete information potentially represented with merged events can be more valuable to advertisers. A use case might be adding a lifetime value attribute in the server event, whereas you might not want to expose that value with in-line Facebook tracking code. If the server & browser events are merged, that lifetime value becomes available for use in Facebook business tools for creating custom audiences & conversions.