Definition: The frequency metric on Facebook is the average number of times that each user saw your ad.
How it’s used: Frequency on Facebook enables you to build awareness by showing you ad multiple times to your target audience. Depending on budget, audience size and campaign schedule, frequency may be one or two or may be significantly higher.
Frequency is an excellent tool and important to monitor, as it can provide insights into whether your audience is experiencing ad fatigue, demonstrated by an increase in frequency as performance drops. If your audience has seen the ad too often, it may be time to change the creative and/or audience targeting.