Here is something that you could do:

1. Write a database trigger so that, when a user signs up for your service, a subportal gets created with their username and gives them admin permissions.  Use some specific default template so that they get, for example the default Blog setup.
2. Purchase a multi-portal signon module (or write one yourself, there's a couple of tutorials on how to do this) and install that.
3. Make all your login links point to the correct login page and use IIS rewriting to make sure that people go to the appropriate login page.
4. Make nearly all your modules except HTML, media, blog, etc premium modules.

I bet there's an easier way to do #1, even.  Or, heck, there's a ton of social media modules available on SnowCovered.  If you're thinking about doing it, chances are that somebody on SnowCovered is selling a module that fits that need or can be massaged into fitting that need.