C., your story is very similar to mine. 2 years ago i had my 15 year old niece living with me. FB wasn't around, but she had a myspace. actually i later found that she had multiple under different names. anyway, one night a boy called the house. i handed the phone to her. happy that she made friends at school since she had changed towns and everything else. after she hung up, i come to find out she met him on myspace and gave him my phone number to call her. it was a scary eye opener for me! at 27 i had to sit and talk with my niece about how dangerous the internet was and how my number was connected to my address and everything else. at 15 she didn't see the potential danger.
going to from toddler mode to 15 year old mode was not easy. i did the best i could. but i treated her how i would treat my children. you are responsible for her actions and life. however she came to be with you, you are responsible. you can't worry about being strict, it's about safety and really teaching her how other people percieve her. being not far from her age, at first i tried to be more her friend so her transition would be easy living with us, but in alot of ways it backfired. treat her as you would your child. your friend is right, lots of kids have them now...but if she is not using it right, then she hasn't earned the priveledge. she is also setting an example for other children in the house. my two year old imitated my niece saying hell. i really got thrown into full fledged parenting way before i was ready. i give you credit for taking care of her...it's not easy. good luck in whatever you decide.