How to keep friend list Private on Facebook?

Adjust who can see your Friends section on Facebook - 

Your profile's Friends section is by default public, which means that anybody can see it.

In order to change who can view your Friends section:

• On Facebook, click your profile image in the upper right corner.
• Click Settings, then select Settings & Privacy.
• Click Privacy in the left column.
• Look under Who can view your friends list. and on the far right click Edit.
• Choose the audience you want to have access to your friends list (for example, Friends).

Remember: On your profile, the only thing you can control is who can view your friends list. On their own profiles, your friends can manage who can see the friendships they have.
For instance, if you choose the Only me setting for your friends list, only you will be able to see your whole friends list on your profile. However, anyone can see that you are friends if your friend has set their friends list to be public.

People will be able to see your friendship in News Feed, search, and other places on Facebook if they can see it on another profile.

When others view your profile, they might notice friends who are friends with you.