I have noticed this on many forums. I think a lot of it is older people maybe retiring etc and the younger people are more into Facebook and other social media. There are some genealogy sites that have become almost completely inactive, with most members now on the FB page. My welding forum has been dying a slow death and issues with the site haven't helped. I belong to a slew of diesel forums and have watched all of them gradually die down to a trickle except for FTE and the Mercedes diesel forum, perhaps because most diesel Mercedes are owned by older people who still use the old fashioned desktop computer for their social media platform. The Powerstroke forums I can't explain why all are down except FTE which is very active, other than to surmise that most left the other forums for FTE.