From my own impression and what other people here are saying, it seems to boil down to ease-of-use: 1) high admin costs of forums due to security issues and sp

From my own impression and what other people here are saying, it seems to boil down to ease-of-use: 1) high admin costs of forums due to security issues and spam, 2) usability issues from clunky interfaces and separate accounts for each forum. On Reddit/FB/etc., the admins don't have to take care of security, and users only need one login to access any community on the site. So yes, the demise of the forums is in some ways quite regrettable, but nonetheless understandable.