Wikipedia's "French Nobility" is a good article. I looked it up in an attempt to understand Proust (and other French novels) better. Here are some things I learned.

Other activities could cause dérogeance, or loss of one's nobility. So were most commercial and manual activities strictly prohibited, although nobles could profit from their lands through mines and forges.

Thus, being a noble had some severe disadvantages.

Other than in isolated cases, serfdom ceased to exist in France by the 15th century....

That sounds like England.

Figures differ on the actual number of nobles in France at the end of the 18th century. For the year 1789, the French historian François Bluche gives a figure of 140,000 nobles (9,000 noble families) and claims that around 5% of nobles claimed descent from feudal nobility before the 15th century.[1]

With a total population of 28 million, this would represent merely 0.5%. The historian Gordon Wright gives a figure of 300,000 nobles (of which 80,000 were from the traditional noblesse d'épée),[2] which agrees with the estimation of the historian Jean de Viguerie,[3] or a little over 1% (proportionally one of smallest noble classes in Europe)....

So little of the nobility actually were descended from the medieval nobility! This, too, doesn't sound so different from England. Maybe the War of the Roses didn't really destroy the ancient English nobility. It seems France was as open, or more open, than England to admitting non-nobles into the nobility. See the next excerpt:

From 1275

to 1578, non-nobles could acquire titles of nobility after three generations by buying lands or castles that had noble privileges attached to them, that is to say that these fiefs had formerly belonged to a noble lord or the king and had been given in feudal homage. Non-nobles could not possess noble fiefs without paying a special tax on them (the franc-fief) to their liege-holder.

In the 16th century, families could acquire nobility by possessing certain important official or military charges, generally after two generations.

Many titles of nobility were usurped by non-nobles in the Renaissance and early 17th century by purchasing fiefs and by living nobly, i.e. by avoiding commercial and manual activity and by finding some way to be exempted from the official taille lists. In this way, the family would slowly come to be seen as noble....

The noblesse de lettres became, starting in the reign of Francis I of France, a handy method for the court to raise revenues; non-nobles possessing noble fiefs would pay a year's worth of revenues from their fiefs to gain nobility. In 1598, Henry IV of France undid a number of these anoblissments, but eventually saw the necessity of the practice.

The last excerpt, below, is useful for interpreting names:

The use of de in noble names (Fr: la particule) was not officially controlled in France (unlike von in the German states), and is not reliable evidence of the bearer's nobility. A simple tailor could be named Marc de Lyon, as a sign of his birth place. In the 19th century, the de was mistakenly adopted by some non-nobles (like Honoré de Balzac) in an attempt to appear noble....