ASP.net would be my second-choice to ruby on rails. It's a close call. If Castle's ActiveRecord was better when I tried it, I would have probably never delved into rails ages ago. I'm glad I did though.

In favor of Rails:

-There are pre-existing FOSS social network apps in rails you can modify.

-You have infinite scalability where your only multiplier is hosting cost b/c there are no windows licenses, so no obstacles to moving into the cloud, virtualizing, setting up a friend

-With rails, you are mostly saved from SQL. Castle's activerecord might do this on .net, but was lousy when I tried it a while ago. Might be better now.

-It is easy to be coddled by ASP.NET and not understand the parts underneath, which works well until the abstraction leaks (viewstate, page layout, etc).

That said, asp.net is not a bad option, and would be my second choice over the java frameworks I've tried, or php. .NET has a nice IDE and better tooling than Rails.