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) was for some years a major nexus of discussion among serious collectors. Although the participants there have moved on to various Facebook groups, the now-dormant site is still full of invaluable information. One of the oldest and largest forum sites is the Fountain Pen Network (FPN). Discussions there are overwhelmingly dominated by users rather than collectors. Older posts may be of more interest, since many former participants have left for Facebook and sites such as Fountain Pen Geeks.
Thanks to the ease with which photos can be shared -- not the case with web forums -- Facebook has become the main venue for collector-to-collector conversation. Some of the leading Facebook groups are: Fountain Pens; Vintage Fountain Pens and Pencils; Waterman's Collector; Antique Writing Equipment and Accessories; Vintage Fountain Pen Collectors -- but there are many more.