The emphasis on the decay beneath the polite, dull society of the Guermantes mirrors the decline of the emotional vividness of Marcel’s existence. Part of this is ascribable to Marcel himself not playing a huge role in the proceedings, but even when he does, there is a lifelessness to it all. Only a few times does he drop hints as to the nature of what is going on, which form the thin backbone of the first half of the book, and a good part of the second. It is this: just as Marcel had earlier found that he could enjoy the company of some silly girls more than any profound conversation with intellectuals, he is here slowly discovering that such high-minded conversation has only the most tenuous link to interaction. Several times, he encounters people behaving strangely with him, and several times he realizes that although he had assumed that their words and actions were in response to his own, it is actually nothing more than a spark escaping from their own solipsistic existence.
Legrandin, dull even by the standards of The Guermantes Way, snaps at Marcel at one point:

“You might at least have the civility to begin by saying how d’ye do to me,” he replied, without offering me his hand and in a coarse and angry voice which I had never suspected him of possessing, a voice which, having no rational connexion with what he ordinarily said, had another more immediate and striking connexion with something he was feeling. For the fact of the matter is that, since we are determined always to keep our feelings to ourselves, we have never given any thought to the manner in which we should express them. And suddenly there is within us a strange and obscene animal making itself heard, whose tones may inspire as much alarm as the person who receives the involuntary, elliptical and almost irresistible communication of one’s defect or vice as would the sudden avowal indirectly and outlandishly proffered by a criminal who can no longer refrain from confessing to a murder of which one had never imagined him to be guilty. I knew, of course, that idealism, even subjective idealism, did not prevent great philosophers from still having hearty appetites or from presenting themselves with untiring perseverance for election to the Academy. But really Legrandin had no need to remind people so often that he belonged to another planet when all his uncontrollable impulses of anger or affability were governed by the desire to occupy a good position on this one. (208)

From this he later concludes that Norpois, who insulted him behind his back earlier, was acting out of similarly unknowable motives. And thus:

What we remember of our conduct remains unknown to our nearest neighbor; what we have forgotten that we have ever said, or indeed what we never did say, flies to provoke hilarity in another planet, and the image that other people form of our actions and demeanour no more resembles our own than an inaccurate tracing, on which for the black line we find an empty space and for a blank area an inexplicable contour, resembles the original drawing. (281)

This is, again, an almost Wittgensteinian notion of speech, yet Proust allows for sudden anomalous expressions, as though the absence of semantics in the translation of internal feeling to external behavior produces inherent imperfections. And as the behavior of Marcel’s associates is more circumscribed by society, so their deviations become more apparent, as they progress from silly teenage exclamations of intuitive feelings to becoming models of decorum. Ironically, this exposes their own preoccupations more, since the deviations are easy to spot.
The most blatant example of this disparity is Marcel’s relation to M. de Charlus. Charlus is, as Proust will discuss at great length, homosexual, and his interest in Marcel is so assaultive and histrionic (not to mention manic-depressive) that the narrative strains credulity in attempting to portray Marcel as fairly oblivious to Charlus’s quirks. (Alain Delon played Charlus in Swann in Love, but hell, Rip Taylor probably could have pulled it off.) But coming after hundreds of pages of genteel boredom, Charlus is vastly entertaining. A sample:

“Let us return to yourself,” he said, “and my plans for you…Given a very considerable lead over your contemporaries, who knows whether you may not perhaps become what some eminent man of the past might have been if a beneficent spirit had revealed to him, among a generation that knew nothing of them, the secrets of steam and electricity. Do not be foolish, do not refuse for reasons of tact and discretion. Try to understand that, if I do you a great service, I do not expect my reward from you to be any less great. It is many years now since people in society ceased to interest me. I have but one passion left, to seek to redeem the mistakes of my life by conferring the benefit of my knowledge on a soul that is still virgin and capable of being fired by virtue…Perhaps in teaching you the great secrets of diplomacy I might recover a taste for them myself, and begin at last to do things of real interest in which you would have an equal share. But before I can discover this I must see you often, very often, every day.” (301)

Though it’s a comical example, it again paints an exchange in which the subtext is, by a huge margin, exclusively within the mind of only one of the participants. Though Charlus has a unique manner of speaking, he has done reasonably well in society, and has somehow kept his own predilections from surfacing in speech.
When Marcel’s grandmother falls to her deathbed pages later, the effect is paradoxical; the grandmother, unconscious and dying, communicates more to her family than all those in high society have up until that point. It’s this irony that lies when Marcel recalls his mother chastising him over his infatuation with Mme de Guermantes:

“You really must stop hanging about trying to meet Mme de Guermantes. You’re becoming a laughing-stock. Besides, look how ill your grandmother is, you really have something more serious to think about than waylaying a woman who doesn’t care a straw about you,” instantaneously–like a hypnotist who brings you back from the distant country in which you imagined yourself to be, and opens your eyes for you, or like the doctor who, by recalling you to a sense of duty and reality, cures you of an imaginary disease in which you have been wallowing–had awakened me from an unduly protracted dream. (385)

This memory isn’t mentioned until the second part of the volume, but since it occurs chronologically in the first part, it serves as the epitaph to his infatuation with the Guermantes, since he shortly goes on the warpath against them.