This is a great question. It might be helpful to summary briefly what the book is about before looking at its cultural significance.
The book, Da Vinci Code is essentially about a conspiracy theory. Dan Brown states that Jesus was married and more importantly he had a descendant. Moreover, this line continues to exists and the Catholic church wants to destroy it.
Based on this theme, there are several social implications.
First, there is a strong suspicion when it comes to the Catholic church in particular and organized religion in general. Dan Brown's book would not have made a huge impact, if people did not distrust organized religion.
Second, the book also shows that from a cultural point of view, people like to question power. And no matter how far fetched the conspiracy theories are, people have a desire to believe in them. Dan Brown's book was nearly complete fiction and so many people still believed. This was remarkable.
Third, this book also shows the fascination of our culture with esoteric knowledge and symbolism.