When you create an ad on Facebook, one of the key factors that determines who will see it is the target audience you choose during the ad creation process. Facebook offers very precise targeting options, meaning you can direct your ads to specific users based on various factors, including demographic data, interests, online behavior, and much more.

However, besides the target audience, there are several other factors that influence who exactly will see your ad:

1. Demographic Data – You can choose to have your ad shown to a specific group of users based on their age, gender, location, and language.
   
   

2. User Interests and Behavior – Facebook allows you to target users based on their interests (for example, sports fans, music lovers, travelers) and their online behavior, such as pages they follow, types of content they engage with, and even past interactions with your brand.
   
   

3. Custom Audiences and Lookalike Audiences – If you have data about your existing customers or users (such as email addresses or phone numbers), you can create a Custom Audience to show your ad specifically to them. Additionally, with Lookalike Audiences, you can target people who resemble your existing customers, which is useful for expanding your reach.
   
   

4. Budget and Bidding – Facebook uses the budget you set for the ad and the type of bid you choose to determine which users will see your ad. A smaller budget may mean your ad is shown to fewer people, while a larger budget can expand your reach.
   
   

5. Device and Platform Types – Ads can be shown on different devices, such as mobile phones, tablets, or desktop computers, and across different platforms within the Facebook ecosystem, including Instagram, Messenger, and Audience Network.
   
   

6. Setting the Time Frame – You can also choose when exactly you want your ad to appear, whether it’s on specific days of the week or certain hours of the day, which can also influence who will see it.
   
   

Finally, it’s important to note that Facebook uses algorithms and machine learning to optimize which users will see your ad, based on their engagement and other parameters. As an advertiser, you can track the performance of your ads and adjust your targeting to improve results.

In short, who will see your ad depends on a variety of factors, and with proper settings and targeting, you can achieve maximum efficiency for your Facebook ads.