Welcome to the Forums and thank you for your interest in ExpressionEngine!
We’ve got a rather large project coming up which will involve some social aspects, monetisation streams (on-site advertising plus virtual product sales and account up-selling), and it just seems like too much of a stretch for WordPress to handle securely and tidily.
I think you are going to be pleasantly surprised with what is available for ExpressionEngine. The key point is that ExpressionEngine is built on CodeIgniter, EllisLab’s world-class PHP Framework. On top of that, ExpressionEngine offers a robust add-on development API.
This adds together to allow developer to create add-ons that integrate seamlessly with EE. You can get a sense for all that is available over at Devot:ee.
To address your points directly, there some add-ons I would like to point out. For advertising, there is Ad Manager, a full featured advertising management module.
Product sales can be handled by your choice of eCommerce systems. ExpressionEngine includes the Simple Commerce Module for use with PayPal. Other options include BrilliantRetail, CartThrob and Exp:resso Store.
For Memberships, again the Simple Commerce Module can sell memberships to your site. However, robust subscription options are available with Membrr.
I look forward to any other questions you might have!