I am an only child. I've heard that only children tend to have a harder time making friends, but I'm not sure that's true.
I am not any only child because i have one sibling but I have also heard they do have a harder time with friendship. 
From my experience, it encouraged me to actively seek out friendship more, so I became more skilled at conversation and meeting new people.
Throughout history, only children were relatively uncommon but I believe that this is becoming more common now with how expensive racing kids are.