"The person who presents himself there before you is Dr. Des Rogis, who, full of rage at not having made a name for himself, has written a book of medical pornography, and willingly blacks people's boots in society, while he is at the same time discreet." (1.7.180)
This information, which is passed along to Frederick at Rosanette's wild party, is typical of the kind of gossip the Sentimental Education folks spend their time passing along. Like Dr. Des Rogis, everyone is a social climber and everyone is a gossip. And of course, everyone's a critic.
And the Sphinx drank brandy, screamed out with her throat full, and wriggled like a demon. Suddenly her jaws swelled, and no longer being able to keep down the blood which rushed to her head and nearly choked her, she pressed her napkin against her lips, and threw herself under the table. (1.7.218)
The party that Arnoux takes Frederick to shows the underbelly of the upper-middle class. These people party to excess, which both excites and repulses Frederick. Which emotion wins out?
[I]t was as if he had caught sight of whole worlds of misery and despair—the charcoal stove beside the truckle-bed, the corpses at the morgue in their leather aprons, with the cold tap-water trickling over their hair. (1.7.253)
Frederick finds these parties fascinating if not a little depressing. He's definitely not in Nogent anymore—but is it Paris or the people that make the party?
Then, anxious to become acquainted at last with that vague entity, glittering and indefinable, which is known as "society," he sent a note to the Dambreuses to know whether he might be at liberty to call upon them. (1.8.3)
Here we see Frederick makes his first push to be accepted by the Dambreuses. It takes him a while to get his whole act together, but he finally makes contact.
Sénécal had been visited by men in blouses—all patriots, all workmen, all honest fellows, but at the same time men whose society seemed distasteful to the advocate. (1.8.98)
Oh, vocab lesson! Society can also mean company—as in this case. And here we get a glimpse of Sénécal's compatriots, a hardcore bunch that on the other side of the spectrum from Frederick and his crew.
[Sénécal] constructed an ideal of virtuous democracy, with the double aspect of a farm in which the landlord was to receive a share of the produce, and a spinning-mill, a sort of American Lacedæmon, in which the individual would only exist for the benefit of society (1.8.99)
Sénécal's ideal society: a socialist environment that doesn't have the kind of hierarchy involved in a monarchy. Instead of a dog-eat-dog world, everyone would take care of each other. As history tells us, though, it's not always that easy.
A land-owner was saying: "This is a class of men that dreams of upsetting society." (1.8.339)
During a party, Frederick overhears some discussion about the class conflict. Those who own property are threatened by those demanding equality and the end of private property. And guess what? Frederick couldn't care less.
"Enough of metaphysics; no more phantoms! There is no need of dogmas in order to get the streets swept! It will be said that I am turning society upside down. Well, after all, where would be the harm of that?" (1.9.80)
Deslauriers always has an opinion, huh? This time, he's explaining his ideals to Frederick. He believes there's too much philosophy and not enough action, and he's seeking some pretty radical power inversions.
A fortnight afterwards, he renewed the same request, and the clerk administered a lecture to him on the extravagant habits to which he gave himself up in the Arnoux's society. (1.5.105)
Frederick's been kind of a crummy friend to Deslauriers, and Deslauriers isn't psyched about it. After all, he's been ditched mostly because of his class status. Plus, the debts that Frederick is accruing are causing financial problems for both of them.