If you pin someone as your #1 BFF, it means that they are your best friend forever. You can do this on social media platforms like Facebook and Instagram. To do this on Facebook, go to the person's profile page and click on the "Friends" drop-down menu. From there, select "Edit Friends." Then, click on the "Best Friends" option and select the person you want to pin as your #1 BFF. On Instagram, go to the person's profile page and click on the "Following" button. From there, select "Edit Following." Then, click on the "Best Friends" option and select the person you want to pin as your #1 BFF.