Although there are other pen-oriented discussion groups, the following have been singled out for number of participants and for content likely to be of interest to vintage pen and pencil collectors. For more, see some of the links listed under Collectors' Home Pages. Note that most online pen forums are dominated by user-hobbyists, and by discussion (and selling) of newly-made pens. Note too that you will often see the same posts on several different boards, which leads to an unfortunate duplication and fragmentation of the resulting discussions. The discussion below is restricted to English-language forums; there are quite a few notable forums in other languages, which we unfortunately do not have the time to follow with any consistency.
The simply-named Fountain Pen Board (FPB) is currently the main nexus of discussion among serious collectors. Until recently, the highest-volume forums were those at the Fountain Pen Network (FPN), though discussions were overwhelmingly dominated by users rather than collectors. The site's recent management issues seem to have cleared up, but many participants have left for the FPB and for the up and coming Fountain Pen Geeks' forum.
Zoss list: An active listserv site run by Tom Zoss. Posts to the site are sent as email to subscribers, but if you are already overburdened with email, you can elect to receive posts in digest form. This is a moderated list, with a good balance of old and new.
Pentrace: Sociable chat, often veering off in directions unrelated to pens. Active, but focus is more on using pens than on collecting them, more on new pens than on vintage. Threaded, lightly moderated. Sunday show-and-tell picture posts have become an established tradition there.
There is also a bewildering array of pen-related pages on Facebook, many of them closed groups. How these will shake out remains to be seen.