Facebook defines Frequency as impressions divided by reach. In other words, "out of the people who saw your ad, how many times did a unique person see it on average".
Facebook also calculates the metric based on sampled data, so it is only an approximation. Read more on Facebook's article.
Frequency Caps for Ad Delivery
Facebook Newsfeed ads have a native daily frequency cap of maximum 1 per 6 hours on Facebook by each page or ad account. The cap is the same for people who have liked the page. This means the people in your audience cannot have your sponsored posts placed on their newsfeed more times during the same day. Over the course of multiple days, the total impression count can be higher.
Right-Hand Column Ads
The Right-Hand Column (a.k.a. RHS) placement on desktop has no daily frequency cap, so ads can obtain far more impressions in this placement than Newsfeed ads. The average CPM on Right-hand column is also substantially lower since the placement is not as noticeable and ads are smaller.
In Instagram feed, a person won't see ads from a single Instagram @handle or app more than 4 times a day In Instagram Stories, a person won't see the same ad more than once every 48 hours and won't see an ad from the same advertiser more than once every 24 hours A person who sees an ad on Instagram feed or Stories won't see an ad from the same advertiser for 6 hours in Instagram feed A person who sees an ad on Instagram feed or Stories won't see an ad from the same advertiser for 24 hours in Instagram Stories
Source: About the delivery system: Frequency caps, Facebook Help article
As your campaigns continue to deliver over course of multiple days, the frequency will begin to rise unless the audience is large enough. It is good to monitor the average frequency of your ads in order to avoid ad fatigue. The more frequently people see an ad, the more likely they are to ignore it, or worse, hide it, lowering the relevance score of the ad and thus hurting the ad's future performance with a higher cost to reach people.
The point at which frequency leads to a drop in performance varies, but you should be mindful of the frequency as it exceeds > 3. When you identify a drop in performance with no other variable changing than frequency, consider changing your creative to combat potential ad fatigue.
- One way to automate this in Smartly.io is using ad rotation, based on frequency. This allows you to automatically switch the running ads to new ones when the average frequency goes over a certain threshold.
- You can also set up custom triggers to pause the ads, which have a frequency greater than the certain number.
Note that Facebook's Frequency metric refers to average frequency among the people in your audience who have seen the ad at least once. Although your ad might have a reported frequency of 3, a person chosen randomfly from the audience might have seen the ad zero times or 10 times (within the daily frequency limits, of course).
Daily Unique Reach Bidding
Some Facebook objectives allow bidding optimized for daily unique reach. With this option, your ads will be delivered to as many people as possible at a maximum of once per day. This should be used when you have a very precise or small audience (and a budget high enough to reach most of the audience). See this blog post for more insights on bidding.