First, Harvard University is a private school, and, as such, may set whatever admissions standards it chooses to set (as long as it is not discriminating on the basis of race, etc.).  Secondly, these applicants exposed themselves on social media - not the brightest thing to have done under any circumstances.  More universities, both public and private, need to use all the tools in their kits to evaluate student applicants.  The use of social media for such screening is completely acceptable.