Compared to a local email archive from mailing lists I think that web forums are suboptimal for collaboration. That’s one of the reasons why I like Collabnet’s discussion services (as in Funambol Forge), that integrate a mailing list with a simple web forum UI: you can have the best of the worlds.
Reading Canonical’s Survey Results I was surprised to notice that their community prefers to use web-based forums to mailing list for community support. Is that because Google indexes forum archives better than mailing list archives? The’ Ubunty forums entry page doesn’t look too inviting, so I imagine that the most common entry point is a search. From there on the user is engaged and starts posting. My feeling is that web forums have a lower barrier to entry and mailing lists have a higher retention rate of contributors. Does anybody have pointer to scientific studies about this topic?