I've seen the term "social prosthetic" used in two ways:
- An external tool for helping with one's lack of social ability
- Another person whose skill fill in for one's own lack
Personally, I refer often to the former - a tool to help prop up my own inherent difficulties with being a social human.
Though, I have often found the latter also be the former - i.e. I make one friend who then is my link to more friends with whom I'd otherwise find myself unable to maintain relationships.