Thanks a lot, folks, I appreciate the feedback. Just to clarify:
I do want users to be able to create blog posts that funnel into a master blog (community blog). Whether they have a place that is their own blogspace is irrelevant as long as they never leave the site. In other words, a social networking site where a community blog is one of the features.
I think what I want is probably Community Blogs for BuddyPress (https://wordpress.org/extend/plugins/bp-community-blogs). The difference, then, would be this plugin means all posts go to a single blog and WPMU means all posts each have their own blog which are all served from within my site?