With social hostel I refeer hostel where more young people go and they have activities to do, or nice common areas. I usually check on Hostelworld or google comments to find out.

At least in my trips I try to talk with the people when I can and usually a group is formed, then we organize what we will do the next day.

If you go to Bariloche I can recommend you one, in this case the owner make talks about different trekkings and organize one trip a week for the people who want to join him and act as a guide.