Advice regarding parenting

I have a younger sister and she lies to us. We have caught her lying a few times now. She goes out with her friends but doesn't tell us. She never lets us meet her friends or give us their phone numbers. We tried to explain to her the reason behind it but she always avoided it. She talks to a guy who she met online through some of his friends, and lies about him too. He's not from her college but when we asked her, she said "he's from class".

We are concerned about her safety and this we have told her in all the ways possible - discussed openly, through bad experiences of others, through news, etc. She always says that she knows all this but again the same behaviour of lying and hiding things.

I don't know what to do.