Revealing identity of anonymous user

I need to write research work about ways how anonymous user identity could be revealed. The main thing that users think that they surf the internet anonymously, but they are entering websites that are using cookies, communicate on social media. So I need methods that are used for revealing users identity while a user is not logged in.

I need to know if user's habits using internet could help identify user's identity.

Maybe anybody has any ideas, that could help me to start writing this research.

Thanks!