The theme of the book Pretty Little Liars is "learn who is most trustworthy in your life".  In this book the girls are bullied by A who knows all of their secrets so they must have told SOMEBODY in their life.  maybe this A person has links to other people that know the girls' deepest darkest secrets and maybe A is having them tell them the secrets for money.  You have to know who to trust in life or else sooner or later everyone will know about everything in your life.  Say you meet a new friend and she is super sweet and caring and never talks bad about anything so you tell her a few secrets about your family problems you really wouldn't want anyone to know. Then the girl goes to another friend of hers and tells her everything.  Then THAT girl tells a bunch of people... and so on. 

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But also in this book, another common theme would be to be careful what you do and make sure no on is watching.  This seems a little creepy but it really applies to this book in general.  The girls not only told all their secrets to people and lie, they also do dumb things and don't look out for who is watching them... like A.