I have only secret facebook with about 10 people who knows who I am, but I really hate it, Im there daily because of updates from websites i like, so it is good for infos, like RSS feed..but I feel like it is less and less lively, what makes me happy.
More than 10 people know who you are, they figured out who you are based on your data.
See
https://upload.wikimedia.org/wikipedia/commons/f/f3/PRISM_Collection_Details.jpg