Sanction refers to either a penalty or a punishment so when you put the word "social" in front of it, it refers to a group or organization that has norms and boundaries and you either get punished from crossing the norms and boundaries, but can also be rewarded for staying and complying with the norm. The reward is also a factor of social sanctioning.

Noun: a friend who is usually only called upon by a friend when a more primary/prominent friend is unavailable.

Taken from the term “second string” in an athletic competition situation. In football, if the star quarterback gets injured during a play, the second string is called off the bench to replace him/her. A “Second String Friend” is essentially benched until needed, if ever.

Synonyms: Plan B, Second Choice, Secondary Friend, Benched Until Needed