I do not think that employers should do this, but that is a personal opinion. Sadly, employees and prospective employees have very few rights to privacy, an area of law that is being carved out on a case to case basis. Some states have been discussing a ban on employers doing this, but I do not know if any statutes have been passed on this matter. On one hand, one can reasonably ask if it is the business of an employer what one does in one's free time. On the other hand, is it unreasonable for an employer to want to know if an employee or prospective employee spend most free time in a drunken state? Is it unreasonable for an employer to want to know if an employee has been spilling some trade secrets or saying bad things about work? Private employment does not provide the right to free speech.
I will say that I always advise my students that this can and does happen. Many people seem to allow complete access to their Facebook pages, for example, which means they do not have a reasonable expectation of privacy at all. And there have been instances in which people have been required to give up their user names and passwords as a condition of employment. Still, it seems foolish to me to put things out there that make a person look foolish in some way, not a good employment prospect, knowing that there is always a chance that this can be seen and judged.