The user ID is often embedded in the URL of a person’s Facebook profile. Look at the URL on their profile page. The numbers at the end are the user ID, like in this example: https://www.facebook.com/profile.php?id=123456789.
Understanding Facebook ID Numbers
What is a Facebook ID number?
Your User ID is a string of numbers that links to your Facebook profile. It doesn’t personally identify you. You automatically have a User ID, even if you don’t choose to create a username.
Finding Someone Without a Username
How can I find someone on Facebook without knowing their username?
The most common way to find someone is through their phone number. Enter the person’s phone number into the search engine. The majority of people have their phones linked to Facebook.
Does Facebook have a user ID?
Yes, your User ID is a string of numbers that links to your Facebook profile but doesn’t personally identify you. You automatically have a User ID, even without a username.