In The Outsiders, why do the Socs ("Socials") hurt Johnnycake, "Johnny" Cade?

Well, you kind of answered your own question there. They hurt him because the Greasers and the Socs, "Socials" hate each other and once in a while they would be in a fight called a "rumble". I wasn't at all surprised that Johnnycake almost died! I mean, Ponyboy says that Johnny's dad beats him up all the time. So there's no surprise there. The Socials jump on other groups in the area too, but not in their home turf. Then one night after Darry slapped Ponyboy across the face because Ponyboy wasn't home yet, Ponyboy ran away leaving Darry to think about what he did. Ponyboy runs to the woods with Johnny and then they get into a fight. Five Socials versus Ponyboy and Johnny. Johnny pulls out his knife after one of Bob's Socials nearly drowned Ponyboy. Ponyboy, now getting violently sick, says to Johnny, "I think I'm going to be sick," and throws up quietly. Johnny had killed Bob, the handsome Social and the rest fled away.

Because they're rich kids who only value themselves and jump on the Greasers in the East side just because they can . . . and yes, they have nothing in common with the Greasers.

I hope this book won't be as sad as it will be in two parts! :)

I think its because they wanted to get revenge on what they did for those two Soc chicks (Cherry and Valencia?). So Bob and his friends get revenge by dunking Ponyboy's head in the fountain. Johnny on the other hand was in no harm. He killed the Soc, Bob, the handsome Soc and the rest fled away. After Ponyboy was dunked, he looks over at poor Johnny, who's face was shed white, complete white, and he said, "I killed him." Ponyboy looks at Bob, the handsome Soc lying in a pool of dark liquid, now spreading quickly. Ponyboy recalls Johnny having a six inch blade in his back pocket, in case of an emergency. So he uses that knife to get rid of the Socs once in for all. 

Then they go on their adventure, AFTER Darry slaps Ponyboy when he got home.