I was previously part of a forum for 6 years when I was a teenager and I made some really good friends. It's understandable when established members of a site form circles or cliques, so to speak, and spend most of their time interacting with only a certain amount of people. But I think getting rid of an entire forum can be detrimental to the community as a whole and alienate a lot of people.
While I personally do not post very often on these forums, I do enjoy reading the various threads. In my opinion, forums are very vital for interaction, especially when new members join the website and are looking for that connectivity with others. It would virtually be very hard for newer members to get involved with a community where the only interaction is within groups or on individuals' feeds; they most likely wouldn't know where or how to begin.
With a forum, it's very open. There are different thread categories with various topics being discussed that give members a chance to be involved with what interests them. After initially getting involved, you start seeing the same forum members, recognizing names, and begin to understand and possibly know others because of this type of interaction. Without a forum, this would go away. Like I said previously, I don't post very often on these forums. However, I have begun to recognize who the active members are, and I've come to understand some of their personalities. If this site's only interaction was through groups and personal feeds, I would feel alienated, and I wouldn't know who anyone was.
I understand that it is very hard to prevent a forum from dying and to keep it's members active. Watching a forum slowly die is painful, I've been there before. But even so, I think a forum in general is still vital for proper interaction among members.