hi i'm a c# developer with 3 years experience, specialized on microsoft technologies.
i will use ASP.net MVC for the backend, with ASP.net Identity as user and role manager, with this framework we can enable socia login and two steps authentications when you consider apropiated (if you want those features).
For database engine i will use SQL Server with Entity Framework.
for the fornt end i will use HTML5, CSS3, jquery and bootstrap so thye site will be responsive.
made it on ASP.net mvc will let me to focus on the security of the web app in an easy way, so i can set the rules for a valid input on server and client side in one single place and let me apply antiforgery tokens, so click on a bad link that can posibly modify data on the app will be rejected.
if you want the app full scalable the developement can be done on ASP.net MVC 6 (that has native integration with Microsoft Azure cloud and is full scalable by default), if not the right answer is ASP.net MVC 5 (becouse the version 6 is on preview).