Social networks can create the illusion that something is common when it is actually rare. Its the local impression that a specific attribute is common when the global truth is entirely different. Eg. In a group of 5 people, 1 person is a drinker and others are drinkers. Everyone in the group is friend with one person and the drinker. So 4 people will see that 50% of their friends are drinkers - even though, in reality only 20% of the 5 is a drinker.