The Soc girl you are referring to is Cherry (Sherri) Valance. She and Ponyboy become fast friends after he and Johnny get Dallas to leave her alone.
While walking home, the two share many things with each other that they wouldn't necessarily have shared with anyone from their own social groups let alone a complete stranger. Ponyboy even opens up to her about his family and "Mickey Mouse" a horse that his brother, Sodapop used to have.
Cherry isn't sure why she is so at ease with opening up to Ponyboy, however he has a pretty good idea. On page 38, Ponyboy thinks to himself:
"She was coming through to me all right, probably because I was a Greaser, and younger; she didn't have to keep her guard up with me."