Last question first: if you've got linux up and running, might as well stick with it. Some folks prefer a *BSD variant but there's at least one AOLserver patch required (there are threads on the topic here), I believe (I'm a Linux user myself).
As far as the rest goes, the toolkit provides most of the tools described at communityzero. The current release version of OpenACS, 3.2.5, doesn't support any notion of subsiting and the user group capability is quite primitive. You could build up the necessary structure without all that much difficulty but a fair amount of programming's necessary. Some of the packages are more "group/scope aware" than others.
OpenACS 4.x, currently available only in as a pre-release snapshot and supporting both Oracle and PostgreSQL, has a very useful package for building subsites. Using this package you can mount forums, shared bookmarks, ecommerce, etc under each subsite and restrict user access to particular subsites.
Not all packages are subsite aware at this point, and we've not completed the porting of all packages from the original ACS 4.2 framework into our OpenACS 4 framework (which differs mostly in that it supports multiple database backends). However we're making rapid progress and people have actually rolled out a couple of sites using OpenACS 4 despite its pre-alpha status.
So the short summary is that our stuff can't be used off the shelf to immediately recreate communityzero.com, but OpenACS 4 in particular is
close to being in a state where it would be fairly easy to build a site of that type.
There are unanswered questions about the scalability of the acs-subsite package - how many subsites and how many users can be supported on a server of given capacity? We don't know at this point.
