Honestly I think generalist forums have been damaged by social network arrival, because social networks are generalist. I think thematic forums have not been very damaged, if you really like sports you should prefer Totallympics rather than a random facebook sports group (and same for any other thematic forum). We are a very small niche, if you just watch football and sometimes some Olympic sports during the Olympics you are probably not going to become a loyal Totallympics user, but I don't think 15 years ago our community would have been much bigger.
By the way, I think welcoming new users is very helpful to make them feel part of the community, when a community is some years old new users may feel like they are going on a boat with no more space available