Whether a secret can be shared between friends depends on a lot of factors, such as the nature of the secret, the parties involved, and the trustworthiness of the friend as a secret keeper. Keeping a dark secret might be fun to some, but it can be stressful to many. Telling it to a dependable friend would help you relieve that stress and she might even offer some valuable advice to help you resolve your problem. But if your friend is anything like Allison in the show, it's probably dangerous to let her know any of your secrets, because she would note them down in her diary, ridicule you like she did to Emily (Shay Mitchell), or even threaten you like she did to Spencer (Troian Bellisario).
The friendship among the five teen girls in Pretty Little Liars is very unbalanced in that Allison never shares her own secrets with her friends. Keeping those dark secrets to herself not only leads to her tragedy, but also puts her friends' lives at risk. I look forward to seeing how the mystery would continue to unravel in Season 2 of the show, which is set to premiere on June 14, 2011!
Disclosure: I have been hired by Warner Bros. WBWord division to promote Pretty Little Liars: The Complete First Season on DVD.