How Do I Know If My Meta Ads Actually Work?

The short version

• —A high ROAS in Ads Manager means Meta found people likely to buy. It does not mean Meta caused them to buy.
• —Retargeting is the clearest case: those people already visited your site. Much of that revenue was arriving regardless.
• —The real question is incremental — how much revenue exists WITH the ads that would not exist without them.
• —The only reliable answers come from holding something back: a holdout audience, a geo split, or a clean on/off test.
• —Airbnb cut roughly $541M of performance marketing in 2020 and traffic returned to about 95% of the prior year's level anyway (see below).

Why this happens

Ad platforms are optimisation engines. You ask Meta to find people who will convert, and it does exactly that — it finds the people most likely to buy. Many of them were already going to buy. When those orders land, they land with a Meta ad in the path, and Meta reports them as its own. The better the targeting, the more of your existing demand it captures and re-labels as new.

The word for what you actually want to measure is incrementality: the revenue that exists because of the ad and would not exist without it. Reported ROAS and incremental ROAS can point in opposite directions, and the channels that look best in the dashboard are frequently the ones best at intercepting demand you already had.

The most-cited real-world example is Airbnb. In 2020 it cut performance-marketing spend by about $541 million (total marketing fell from roughly $1.14bn to $482m), and its traffic still returned to approximately 95% of 2019 levels without that spend — which told them the brand, not the bidding, was doing the work. Figures are from Airbnb's FY2020 reporting as covered by Marketing Week and Campaign in 2021.

What you can do about it today

1. 1

   Run a holdout

   Exclude 10-20% of your audience from Meta ads for two to four weeks. Compare revenue per person in the held-out group against everyone else. The gap is your real lift. This is free and it is the single most useful thing on this page.

2. 2

   Separate prospecting from retargeting

   Report them as two different things. Retargeting ROAS is almost always flattered; prospecting is where genuine new demand is created. Judging them on one blended number hides both.

3. 3

   Watch MER, not just ROAS

   Total revenue divided by total ad spend across all channels. If platform ROAS climbs while MER stays flat, the platforms are trading credit between themselves and nothing new is being created.

4. 4

   Try a geo test if you cannot hold out an audience

   Pause Meta in one comparable region and keep it running in another for two weeks. Compare total regional revenue. Less precise than a holdout, but it works when audience splits are impractical.

How Causality Engine helps

If you would rather not run experiments by hand, Causality Engine reads causal lift straight out of your GA4 export — per channel, with confidence intervals, in 5-10 minutes for €99. You get the incrementality answer without pausing any campaigns.

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