(asked by Anne from DC and -I think- currently in France)
The deal is pretty simple. Not every noble died during the Revolution (Unfortunately? No, I didn’t say it). The Revolution was not about genocide but about democracy.
First, a common misconception abroad (and even among some uneducated French people) is that the guillotine’s main purpose was to kill the nobles.
Well, if it had been the case, why bother with a guillotine? Torching their houses and gutting them would have been more efficient (or at least faster).
No, the guillotine was just the “tool” to execute people that were sentenced to death.
But at the time, the surest way to get a one-way trip to the guillotine (apart from the obvious: mass murder, etc.) was to be labeled “Enemy of the Revolution.”
That included members of the aristocracy, but not all of them, some were actually in favor of the Revolution (the most famous one being the Marquis de Lafayette). That also included people from the commonalty, especially once Robespierre had taken over and lost it and started to sentenced to death anybody that would disagree with him (until he was deposed and sent to the guillotine).
So, even today, there are living members of the aristocracy in France.
Thing is, that the aristocracy and the titles that go with it have no legal value anymore. But some people, because of the importance of lineage and tradition in those families, or because they still hope to go back to the Monarchy one day (there are some of those in France) kept their title even if nowadays they have the value you want to give them (great value among themselves, no value whatsoever for the rest of the people).
Keep in mind that it’s not the whole aristocracy that does that, just a few (most of them located in the 16th arrondissement of Paris and rich suburbs like Neuilly or Versailles). I had a friend in college that was technically an aristocrat, but his family was your typical French middle-class family, and he couldn’t care less for his noble origins.
While I’m at it, be aware of two things:
Not all people from the aristocracy are nobles from the Ancien Régime. The two Napoléons created their own aristocracies, naming baron, duke, marquis, etc people that were close to them and that were “deserving” for various reasons (military success, financial success, personal favors done to the power in place, etc). I’m not sure what the relationship those “new” nobles have with the Ancien Régime ones, but I suspect the latter to despise the former. And yes, they know who is who, they care that much about ancestry and lineage.
This is also why if you proclaim yourself Marquis of whatever, you can fool a normal French person, but you won’t fool any real aristocrat.
Also, not everybody with a “de” in their last name has noble origins. If the “de” is attached to the rest of the name (Dedieu, Debord, etc) it’s definitely not an aristocrat name but an old French name from the Middle-Ages (for example, Debord means “from the bank” of the nearby river), even though at times, especially if there are many syllables after the “de” the family was an aristocrat one, but attached the “de” to the rest of the name in order to not be seen as such (because they were in favor of the Revolution, because years later they didn’t affiliate themselves with the nobility anymore, etc.).
We also have families that for one reason or the other want to appear noble and managed to attach a “de” somewhere in their last name.
The most famous example is the one of former president Valery Giscard d’Estaing whose ancestors were just the Giscard family but because there was a “d’Estaing” at some point in the family (he seems to have a quite complicated family tree), his dad and other members of his family legally changed their names to Giscard d’Estaing in 1922.
On the other hand, Charles de Gaulle was from an real aristocrat family, even though a very minor one.
And while we’re at it, former Prime Minister Dominique de Villepin whose real name is Dominique Galouzeau de Villepin, is not an aristocrat either. In the 18th Century, his ancestor Mr. Galouzeau married Miss de Villepin (who was I assume from the aristocracy). It’s one of their great-grandson who legally changed his name Galouzeau to Galouzeau de Villepin in the 19th Century, maybe during one of those times when it was “useful” again to be a noble. The name stayed this way for the next generation.
Why does the former Prime Minister go by “de Villepin” and not “Galouzeau de Villepin” or just “Galouzeau” is unknown to me, but I promise to ask him if I ever met him.
I hope I’m answering your question, and if you have any further ones feel free to ask.