PSG thrashed Bastia 4-0 at home on Saturday to maintain its unbeaten start to the season.
After 10 league games, PSG has 24 points the best start since Lyon tallied 28 from 10 in the 2006-07 season.
Ferreira Carrasco opened the scoring in the fifth minute when James Rodriguez set him up from the left and he finished powerfully from close range. He added another after Brazilian defender Carlao's back pass to the goalkeeper fell short.
"Our two goals were marvelous and then 'click' it's all over," Monaco coach Claudio Ranieri said. "That's football."
Sochaux's goals came from corners, highlighting Monaco's vulnerability on set pieces.
Rafael Dias took both corners, the first from the left as Bakambu got the better of the Monaco defense, and the second from the right as Lopy headed home at the back post after losing his marker.
Monaco's top scorer Radamel Falcao missed a late chance to win the game when his mishit effort rolled wide of the post.
Monaco center half Eric Abidal pulled out moments before the game after complaining of a thigh problem.
"It's nothing serious," Abidal said. "I'd already felt some pain yesterday and I didn't want to take any risks. It's better to sit out for three days than three weeks."
Saint-Etienne climbed up to seventh place after a 3-2 win against Lorient.
The home side raced into a 2-0 lead after 14 minutes thanks to goals from Brazilian striker Brandao returning from a thigh injury and winger Benjamin Corgnet.
Yoann Jouffre pulled a goal back for Lorient shortly before halftime but Max-Alain Gradel restored Saint-Etienne's cushion with 20 minutes to go.
Vincent Aboubakar pulled a late goal back for the visitors.
Also, forward Jimmy Briand scored in injury time as 10-man Lyon salvaged a 1-1 draw at home to Bordeaux.
In a dull game of few chances, midfielder Ludovic Obraniak put the visitors ahead in the 62nd when he volleyed in a cross at the far post after some poor marking from Lyon's defense. Bordeaux striker Cheick Diabate missed a free header moments later.
Things got even worse for Lyon when Alexandre Lacazette was shown a second yellow card and sent off in the 66th.
Briand missed a late chance to equalize when his header was placed straight at goalkeeper Cedric Carrasso.
But he made up for that in the dying seconds when he slotted home the rebound after Carrasso had saved Nabil Fekir's shot.
"We dropped off too much and it's unbelievable to concede a goal like that," Bordeaux forward Henri Saivet said. "Everyone has to do their job and, I'm sorry, we didn't do that."
Despite salvaging a point, Lyon is in 11th place and has won just one its past 12 games in all competitions. Bordeaux is in 15th spot.
"We're playing with the means at our disposal," Lyon coach Remi Garde said, referring to the high number of young, homegrown, players in the team. "Some of them don't have much experience and we ask a lot of them. I can understand sometimes that the bar is set a bit high."
Things are unlikely to get easier for Lyon, which won seven titles in a row before Bordeaux ended that run in the 2008-09 season, with trips coming up to Monaco, Saint-Etienne and PSG.
"We need to get some of our experienced players back," Garde said. "We all have to stick together."
