When your relationship is new, you'll spend a lot of time getting to know each other. This will probably include the stories your friends have already heard a thousand times. If you're already getting comfortable with each other, you may have also shared some deep personal secrets with each other. Those deeper levels of trust are an essential part of your intimate connection with each other.
If your partner shares a secret that you're finding hard to keep to yourself, you may be tempted to share it on Facebook. Even if you restrict who can see it, though, it's still quite an announcement when it's seen by 50 or more of your Facebook friends at one time.
Even if you share your message privately to just one trusted friend, you can't guarantee it won't get passed on to yet another trusted friend, who passes it on to another and so on. Remember that Facebook is an almost endless web of social network connections.
That same message might eventually get back to your partner, too. By the time your partner learns you violated your mutual trust, it may be too late to mend the damage. As a precaution, when it comes to your partner's secrets, just avoid typing them at all. If you still can't help yourself and just need to get it out, keep a hand-written diary or journal offline that remains safely in your possession.