I guess people use social media, which is ok for a single response, but a chocolate teapot as a source of coherently assembled opinions that are organised along the thread topic. I presume the internet has so much info available that many people feel they have all the info they need.
What is obvious is the lack of visitors asking questions, which seems counterintuitive given the significant increase in collectors of modern coins. At any coin fair you will see a reasonable number of kids going round the tables. I presume they get all their info from social media. Whilst you can reasonably expect the majority of collectors to be middle aged and up, the vast majority of collectors do have access to the internet, so why they don't interact with others is beyond me.
A parallel problem to that of forums is happening to many numismatic societies. The South Manchester decided last meeting to only have one meeting a month compared to 2 a month in autumn and spring. The reason - numbers have dropped off such that there is no longer a critical mass of members to fill the schedule with talks. There is a near total absence of younger people coming through. At the last meeting, I was the spring chicken, and I get a state pension.