Mapping a Twitter like domain with Fluent NHibernate

I'm currently learning NHibernate, and one of the best ways to learn is to blog about it :) Since I'm not a big fan of XML files, I also wanted to use Fluent NHibernate to do the mapping. As an example I will use a social messaging domain, similar to Twitter. I will also use a Top-down approach, starting with the domain model, writing the mapping, and finally creating the database schema using the SchemaExport class in NHibernate.