It is true that the 20 something crowd doesn't use Facebook like they did 15 years ago, but the platform seems far from dead. I enjoy keeping up with friends, their adventures, how their kids are doing. The cat videos are really fun also, LOL. I've never had a paranoia about privacy, and I've never had a problem, it's just not an issue. I've never used Facebook marketplace, so I don't know how that works.
Some of the groups I follow are Amtrak's Empire Builder, Amtrak's Coast Starlight, Breviary and Divine Office Discussion Group, The Librarianologist, Funeral Home & Mortuary History, Bicycle Touring for Beginners, Frank Lloyd Wright Nation, St. Patrick's Parish, and several others. The ones I've mentioned typically have several posts a day ... VERY FAR FROM DEAD. Some of the groups you have to join before you can post or even see who is a member. Some, like The Divine Office group will let one ask questions as an anonymous.
I agree that a Facebook profile doesn't work well as a commercial or organization web site, unless a business or organization regularly posts new content or events. Unless one has a Facebook account and is logged in I don't really see the point of trying to use it, but I glean a LOT of useful information through the groups I'm a part of, and in that regard it can be a lot more convenient than a string of e-mails.