Both MVP and MVC patterns are capable of generating enterprise class applications. While MVC is definitely a much better pattern, it is also true that as of today the productivity benefits that you get with the MVP approach is far superior. MVC pattern adds a significantly steep learning curve for the current web developers or even the new ones who are just starting.
Given the expectation of rich user interface for a social networking site, using MVC also additionally means lot more work towards creating custom controls for almost every display requirement. MVP allows us to work with the large number of ASP.NET server controls.
When starting on a new application you can argue more convincingly in favor of MVC. However in our case, since this is an update of the book and the associated source code is building on top of the previous edition, using MVC would have meant a significantly long cycle.
There are no migration wizards available today that help in easily taking some MVP based code and converting it to MVC. Had we selected MVC, it would have meant a rewrite of the presentation layer and would have impacted the release timelines of this book.