Well, it depends on the type of relationship I have with the person who tells me the secret. Also if it involves somebody else, who I care about, I won't think twice in telling that person the secret.
It also depends on the importance of the secret and the type of the secret. If it is something very important like "I killed a man, please don't tell!" of course I will call the police even though I promised.
I usually keep secrets up until the person that told me the secret upsets me and then I tell everyone else.