Reb Moshe, in Igros Moshe EH 4:60, paskened that it is assur to talk to the opposite gender in a social context, and assur to be friends with them, and even seeming platonic relationships between boys and girls are assur min hatorah. In the Sefer HaChinuch, mitzvah number 188, it states there that it is assur for boys and girls to talk to each other.
