I think letting go is hard for most concerned parents, so consider yourself normal. You say your daughter is very social, but didn't mention if she has a record of being very responsible. Does she generally do well in school. Do your know her friends and their parents. Do her friends come to your house for get-togethers? If she has a cell phone are you aware of who she calls and gets calls from.Does she have a my space page- what goes on there? Who pays for gas and car insurance. Remember she is 16- only 16. I'm sure she feels much older. That's normal too. With privledges come responsibilities... at ANY age.
On a school night, with homework done and all classes with average or above grades- 9 PM is a reasonable curfew. On a weekend 11 PM would seem plenty of time to enjoy a social life. Where she is (at all times) and who she is with (at all times) is expected of a parent. Interesting to think that her friends know who she's with and where she is, but she doesn't think it's important for you to know. She is 16 -the conflicts you're having go on in homes across the USA. It is expected that she will rebell and challenge you on what she will consider "tracking" her.
Just a thought-When you go somewhere (or your husband- if you're married) you probably let her know where you will be... out to dinner, at a movie with husband or shopping with a friend, at the grocery store. If you have a cell- you wouldn't be upset if she called you. You'd likely give an extimated time you'll be home. etc. Expect no less of her than you, an adult and parent, would do. I'm sure she feels grown up (that's normal) but these are the times that will challenge both of you. Remind her that she can follow your rules and expectations or expect to be severly stifled with the loss of car keys and staying home rather than be out with friends.
One final thought.. explaining the house rules should not be a fight. So, don't raise your voice, don't get into a shouting match. If she has a job, she wouldn't argue with her boss about the work rules and regulations. If she did she would be fired. Just set the rules and consequences in a very mild mannered, matter of fact tone of voice. When she leaves the house ask the important questions and modify her plan if you are not comfortable with it. Be sure she understands the expections before leaving the house- without ATTITUDE.The rest is up to her.And finally- be a person of your word ALWAYS FOLLOW THROUGH with compliments and consequences.