It's surprisingly easy, with the amount of personal information that people put out there. One of those trashy cheer tumblrs made the mistake of putting her email on her page, which featured her real full name. Based on that alone, I could have easily deduced which was her twitter, and from there her facebook and other social media. Where she was from, probably where she cheered if she did (because all that info is scattered across every platform). It's terrifyingly simple and takes less than 10 minutes. It's not something I ever do because I don't care- but there are people who do and their 'targets' make all that info readily available.
Also depending on who they are, the industry can be very small. When it comes to some of the bigger players, people talk.