Just remember back to when you were that age....although we did not have mobile phones, facebook etc...we too giggled and carried on behind closed doors. They are just being.....teenage girls, and rather behind her closed bedroom door than being out somewhere or other. Just make sure you chat about the pro's and con's of facebook and not to chat to anyone they do not know, as at this age, believe me......they tend to be very trusting. I pop into my kids rooms often, with offering them chips, juice etc....so an excuse to see/hear what is going on, and normally.......nothing much, just the little bitching about a fellow girl at school.....which is also normal, not nice, but it happens, or the latest fashion, hair, make up etc........
Good luck, you are just a loving Mom, ain't nothing wrong with that!!