What Meta puts at the ad set level
Every Meta ad has three parts: a campaign, an ad set and an ad. You choose the objective on the campaign. On the ad set you build the audience, set the budget and schedule, choose placements and, optionally, set a bid. The ads inside it carry the creative. A campaign can hold several ad sets with different audiences, schedules and budgets, and an ad set can hold several ads. (Meta Business Help Center)
The performance goal is also chosen per ad set, and it can differ from the campaign objective: Meta's example is a Sales campaign with an ad set optimized for link clicks. It is the optimization event the delivery system bids on. The one budget exception is Advantage+ campaign budget, which replaces individual ad set budgets with one campaign budget that Meta moves toward the ad sets with the best opportunities, not necessarily evenly.
Why the ad set is where learning happens
Meta's learning phase is measured per ad set. An ad set exits it once it can deliver stably, which Meta says usually takes about 50 results in the week after its last significant edit. Every extra ad set aimed at the same people splits those results and can put your own ad sets into auction overlap. Meta's advice is to avoid high ad volumes and combine similar ad sets. That's why we run fewer, broader ad sets, and why two ad sets in one campaign is not an A/B test.
LinkedIn and Google
LinkedIn now uses the same word: what it used to call a campaign is an ad set, inside what it now calls a campaign. A LinkedIn ad set holds the objective, audience, ad format, placements, budget and schedule (unless the campaign uses Dynamic Group Budget), and its objective and ad format can't be changed after launch. Google Ads works differently: an ad group holds ads and keywords that share a theme, while the budget and most settings sit on the campaign. More in our LinkedIn Ads guide and Google Ads campaign structure guide.
Common questions
What is the difference between a campaign and an ad set on Meta?
The campaign holds the objective, such as Sales or Leads. The ad set decides who sees the ads and how they run: audience, budget, schedule, placements, bid and performance goal. The ads inside it hold the creative.
Is the Meta budget set at the campaign or the ad set level?
Either. By default each ad set has its own budget. With Advantage+ campaign budget you set one campaign budget instead, and Meta distributes it across ad sets in real time, which may mean most of it goes to one ad set. Meta says it suits campaigns with at least two ad sets.
How many ad sets should a Meta campaign have?
As few as the test or the audiences genuinely need. Meta's learning phase guidance says that with many ads and ad sets the delivery system learns less about each one, and recommends combining similar ad sets so their learnings combine too.
Is a LinkedIn ad set the same as a Meta ad set?
Close. Both sit under a campaign and hold the audience, placements, budget and schedule. On LinkedIn the ad set also holds the objective and ad format, which are locked once it launches. On Meta the objective lives on the campaign.