alternative to privatemsg -conversation nodes

I have been looking for it but couldn't find a discussion on substituting node creation and access management for the privatemsg module.

Today I pondered for a few hours more on whether I should start building my brand new site on Drupal 6 or 5. Messaging is something I have to have, the site being a social network, but I see there is more to wait.

I am not a developer at all but when I saw this module (coherent access), I started thinking whether something similar could be used to build another way for people to communicate.

People would be creating node types (of "conversations") and include users so they are able to make changes to those nodes (could be limited to adding comments but it could get more interesting). An inbox would be a listing of nodes a user was invited to with the node items would show the whole conversation (facebook-style conversations seem to be preferred for single messages). Sent messages could show the nodes authored by the user etc.

People on privatemsg support forums ask for features that mimic facebook and other social networks, some, things like adding avatars, notifications whether online etc. I imagine that with messages being nodes, flagship modules like views, panels and cck can be used to make things interesting.

But perhaps we wouldn't want to? Performance-wise, security-wise...

I hope I didn't make a fool out of myself. I am a philosopher by education, spending countless hours trying to figure out drupal development...