Today I came across with this doubt and I couldn't find any documentation or recommendations about it. What do you think/do? The user authenticates with facebook to my web, I want to use it's profile picture as the user's profile picture in my app. Should I use the "url" or should I be downloading the photo to my server to keep it?
You should never use the CDN URL, Facebook does not guarantee that it will always be valid. The best way is to download the image and store it on your server.