Holden is alienated from more than just the individuals he knows: he is alienated from his society as a whole. An example of his disenchantment can be seen in the following quote from chapter 9, about his expensive hotel in downtown Manhattan:

I didn't know then that the goddam hotel was full of perverts and morons. Screwballs all over the place.

These are not people he knows: he is making a comment about his society as a whole being filled with undesirable people.

Holden goes on to implicitly critique the social order in the following quote, also from chapter 9. His bellhop is an elderly man:

what a gorgeous job for a guy around sixty-five years old. Carrying people's suitcases and waiting around for a tip. I suppose he wasn't too intelligent or anything, but it was terrible anyway.

This bellhop is not an individual Holden knows. Holden feels bad for his situation on principle, criticizing a social order that would allow an older man to do heavy and demeaning work ("waiting around for a tip") rather than providing a decent job or retirement for him.

All of this accords with his disillusionment with his school: it is not just a few people, but everyone, from his roommate to the phony, Jesus-loving alum who runs a funeral business, who alienate Holden. For him, the world is filled with self-absorbed phonies who have built a society that doesn't adequately protect the vulnerable people of the world, such as nuns, the elderly, and children.

In The Catcher in the Rye by J D Salinger, Holden Caulfield, the narrator, is a conflicted teenager who has been expelled from yet another school; considers himself an "illiterate," although English appears to be the only subject he is really good at and is depressed and alone; different from his peers and dismissive of all adults who lack sincerity in everything they do. He tries to think of someone to connect with, after arriving in New York, but, in the phone booth, he is unable to think of anybody feasible whom he can call. His brother will be too busy; his sister will be in bed and he's really not in the mood for talking to any adults.  He even realizes that he doesn't like Carl Luce so why call him? After twenty minutes, he just leaves the phone booth after "not calling anyone." This intensifies his isolation.    

In an effort to remove himself from the hypocrisy that surrounds him, Holden does seek new friendships; presumably with people who feel alienated like he does such as prostitutes and from people whose social circles are diverse and can cross barriers, such as bartenders and cab drivers. When inviting the cab driver to drink with him, Holden reveals his desperation for companionship, lying that he is "loaded" and can buy cocktails. It does not have the desired effect though and Holden continues to feel unfulfilled. His meeting with the girls in The Lavender Room confirms that he is disconnected as he suggests that, "I certainly must've been very hard up to even bother.” He seems unaware of the the source of their amusement. Their "hysterics," he feels, are from their own stupidity and not necessarily from his ineptitude or his inability to get a drink. it seems there isn't any society which would accept Holden for who he is.   

Holden always looks for excuses for not getting things done. He can't even pray unless he is in the mood. He is not sure that he believes in God, even though he "likes" Jesus. Holden's life is full of disappointments and "phony" adults always let him down. Even Jesus could not rely on the disciples; he suggests:

They were all right after Jesus was dead and all, but while He was alive, they were about as much use to Him as a hole in the head. 

Holden has the same problem and cannot rely on anyone so it is perhaps better to avoid the problem altogether. It is not just that others alienate him, he alienates himself.