What do you mean by contact via social media?
Some schools have school based social media used specifically for learning. As well, some teachers have familial relationships with some of their students (friends with their families and the like). In addition, a lot can be said for student/ teacher engagement on these levels. Allows for more contact/ support.
The real question may be in the realm of 'what kind of oversight should parents keep to maintain propriety of communications between their children and adults on the internet'. Do we have to regulate whether or not a teacher can walk up to a student outside the school day, or is the onus on the parents to keep those kind of contacts appropriate?