A big part of it was paid off by the revolutionary government seizing and auctioning off Church property (the quantity involved here is massive, so much so that it radically transformed French society by creating a new middle-class of people who were able to buy these "national goods" and turn them into sources of long-term revenue). Another melted away when the government began running the printing press and causing hyperinflation, especially under the Directory. And yet more was paid off with Napoleon's war loots.
Of course, then Napoleon lost and it was France's turn to be looted, so by 1815 it was back to square one with a massive debt. But that one was paid off by more boring means like sound fiscal management and skillful diplomacy.
So it was eventually paid off in the long run but it probably weakened the country coming in the late 19th and 20th century.
