I think part of the retention issues posed here have to do with the platform involved. I'm now past my rookie season as a member here, and I can attest to the quality of this forum. It is a welcoming atmosphere (as far as you can being online), and people are prompt to answer your questions and put thought into their responses. The reason, however, that it seems more people engage in a Facebook group as opposed to an online forum may be accesibility.
First of all, in terms of what pen turning involves, the online forum platform is much more efficient at disseminating knowledge and collaboration as opposed to a massive chat timeline - in my opinion. I would MUCH rather try to search for how to create a Celtic Knot tutorial on this forum as opposed to trying to find it on a Facebook group. I don't believe that Facebook is better than this forum as a space for penturners. However, this does not answer the question, "Why does it appear that there is more user engagement on a Facebook group?"
I have no data whatsoever to back up this claim, so feel free to ignore it completely, but I believe it comes down to the accesibility of Facebook and smartphones. A lot of people have smartphones, and a high percentage of those people already have the Facebook app. It is quick and easy to pull up anywhere you are. Whenever something happens, you get a push notification telling you to open the app.
I also have no data whatsoever on what devices the users of this forum use to engage, and how often they engage. I would pose that a user who only uses the web browser to use the forum, on average, would have much lower engagement level than say the user of the mobile app. I am aware of the IAP iphone app, but there is no app for Android users. However, I use the Tapatalk app for the IAP forum since I am an Android user and I find it to be a very smooth experience. This forum is very accessible, as long as you are using the right tool.
Perhaps one thing to think about would be promoting (especially to those in the Facebook groups) how EASY it is to interface with such a quality forum. Once people discover how much they can benefit from this forum, and how easy and simple it is to connect, then perhaps you may see a higher retention rate.
Once again, this is just a VERY unqualified opinion, but thank you if you made it down this far!