How do I hide my friends list on Facebook so other people can't see who I'm friends with?

Go to your profile > Friends tab > the three dots (or pencil/edit icon) > 'Edit privacy', and set 'Who can see your friends list?' to 'Only me' (or 'Friends' if you just want it hidden from the public). On mobile it's Menu > your profile > Friends > the three-dot menu > Edit Privacy. Worth knowing: even set to 'Only me', a mutual friend can still see you two are connected when they look at their own friends, you can't hide individual mutuals. But it stops anyone browsing your full list. While you're there, check 'How people find and contact you' to lock down who can send you requests too.