If its called a secret why would people tell their friends the secret? Maybe each person have their own definition and ways to keep their secret.
For me, I would share some of my things with my best friend. Not all, but some! Even though how much you trust your friend I believe you will keep some of the secret; the important ones, the things that you shy to speak out and etc. This is because you do not 100% trust your friend! Or maybe you just do not want people to know bout it. You save it for yourself. I am 1 of this. I will tell my best friends a lot of things.. (only my best friend). I shared, I discuss but still there's some I keep it for myself. Just like my friend. She told me lot of thing but still she will keep it for herself. Make sure that nobody else will know. I been shower with lots of secret nowadays. I'm glad that I still keep it for my own. When a friend tell you his or her secret that means he or she trust you. So as a friend of your friend, you should keep this secret. So I'm still keeping. And I hope I won't tell any of it to anyone..