From your post, I cannot figure out what purpose this collaborative
media should accomplish. Are you to develop a product? Probably not,
as your analysis doesn't rate the options in that respect. But if you
are, you shouldn't underestimate the power of mailing lists. Perl5 is
developed on a mailing list. And MUAs have a superiour user interface
compared to websites (better threading, score files, your favourite
editor, keeping track of what you have read, filing, etc). Usenet is,
IMO, also a preferable media over websites.