You can choose to allow only website members to see your Profile or to have your Profile open to the public.  If your Profile is open, it's really open to everyone who has access to the internet, including search engines and people who may not use your personal information in your best interests.  There is some built-in protection.  Only you and the website administrators can see your email address, and your street address, phone number, and birthday are not open to the public.  Otherwise, anyone can see anything posted on your Profile if you decide to leave it open. 
To limit your Profile to website members only, click on Edit Profile and check the Profile Visibility box near the bottom of the page.  Don't forget to save the change.  The box looks like this:
Profile Visibility:  
  Allow only fellow Classmates to view my Profile details. This will block the public and search engines from accessing your Profile. Leave this box unchecked if you wish to allow the public (friends, co-workers, family members, etc.) to view your Profile details.
You can see whose Profiles are closed to the public by clicking on Classmates and Member Profiles after you log out.  A padlock icon appears next to the names of people with closed profiles.  To see which site pages are closed to the public, click on the links on the left when you are logged out.