Bizwomen has released a piece on the best practices for companies (as well as practices to be specifically avoided) with regard to monitoring the social media accounts of employees and job applicants.
According to the authors, there’s nothing wrong with looking at applicants’ social media so long as employers don’t use any information obtained as a result to discriminate against the applicant, especially as it pertains to any membership the applicant may claim among legally protected groups defined by religion, race, or gender, for example.
As to whether or not companies should monitor employee social media use on an ongoing basis, the article contends that doing so is counterproductive and will likely only open up the possibility for aggrieved employees, present and/or former, to claim discrimination as a result.
You can read more about these topics here.