I'm torn on this subject. On the one hand, employers shouldn't be allowed to judge an applicant based on the things they enjoy in their free time, as I feel this infringes fundamentally on a persons individual rights and freedoms. As long as a person isn't breaking any laws I'm not sure what they get up to at the weekend, in the evenings, or with their friends is of any relevance. That being said, if a person is applying to an animal testing lab and their social media presence demonstrates a high level of animal rights activism, then it's fairly reasonable to conclude that their motives for applying may not be because they fancy dripping perfume into bunnies' eyes (as an example. FTR I strongly oppose animal testing).
At my recent interview for 'MASSIVE INSTITUTE' I noticed that despite having not applied via Linkedin, the interviewers had copies of my linkedin profile so clearly they had researched my social media presence. Luckily for me I'm incredibly boring and my facebook feed is just full of photos of my pets and various garden projects and the occasional culinary masterpiece.