As a general rule, the Internet is open access, so unless information is posted on a secure site with restricted access, anyone can view it. Discretion should be used when deciding what information to reveal online as even secure websites can leak data or have it stolen.
Identity theft can easily occur if someone shares too much data online. For example, things posted on social media sites could be used to answer security questions and gain access to more sensitive data. It's best to have profiles set as viewable by only trusted offline acquaintances. Also, financial information should never be given out without ensuring that the site is encrypted.