Learn how to find the Facebook User ID (or “User ID”) of a third party on Facebook.
Steps

Step 1. Go to https://fr-fr.facebook.com in your Internet browser
To find the user ID, you are going to need to use a computer's Internet browser.

Step 2. Log in to Facebook
Enter your email address or phone number and your password in the reserved fields at the top right of the screen, then click on Log in.

Step 3. Go to the profile of the person of your choice
You can find this person by typing their name in the search bar at the top of the screen or by clicking on their name in your friends list.

Step 4. Right click on a gray area of their page
You can see these areas on the left and right of their profile. A short menu is then displayed.
If the computer you are using does not allow you to right click, press the Ctrl key on your keyboard while left clicking

Step 5. Click View Source Code
The source code of the page will then open in another tab.
- If you don't see the option View source code, look for a similar wording, for example Show source Where Source code of the page.

Step 6. Do Ctrl + F (on Windows) or ⌘ Command + F (under macOS).
The search field appears.

Step 7. Type profile_id in the search field
Then press Enter (in Windows) or Return (in macOS). The user ID will then be displayed to the right of the profile_id text.