What is the effect of this naming? What are its social implications?
Austin used names in the proper way............... surnames denoted social class, and this is the way it would have been. I'm linking the following article for you below; you might want to check it out!
The British System of Aristocratic Honorifics
Its an in-depth character map of the play. Now if you see the map, it clearly describes the relation of each and every person of the book. If you notice there are two people with their last names as DARCY. one is FITZWILLIAM and other is GEORGIANA who happens to be a female. But the no. of characters with their last name as bennet are 7 in total(including the parents). An explanation to Why does Austen call Elizabeth by her first name and Mr. Darcy by his last name throughout the novel can be, because if she calls Elizabeth as Ms. Bennet than the readers may get confused with the five sisters, whereas there is only one male character with the last name as Darcy, so she refers him to Mr. Darcy. This is simply because the writer thinks from the writing point of view and the editor thinks according to the readers point of view.
