It's an interesting topic. Going by the stats TC hasn't got any quieter over the years in terms of traffic (It is down over the past couple of months due to lack of news, but that will bounce back). Scroll down to the bottom of the forums and you'll always see a large number of "lurkers".
The main reasons TC has become very bike focused is partly down to a very knowledgable and helpful user base (for which I thank all contributors) and the work that the forum software providers have done in search engine optimisation. If you have a problem with your bike, do you go to Facebook? No. You go to Google with your problem and you will always find TC very near the top of the results, and more often than not the top result. It's the gap in the market that TC is filling very well. There is no other resource like it for Trials.
There has been much debate about the place of forums in the world of anti-social media and there is absolutely a place for them. If you want arseholes, self-professed "experts" on every subject under the sun and pictures of people's cats, head for Facebook. If your topic is of a more important nature, use a forum.
Interestingly, I did some research a few months ago as I was working with a charity that had abandoned their website and gone 100% Facebook. Only roughly 65% of people in the UK use Facebook so that was over a third of the potential donating public they were completely ignoring. The core Facebook platform is also in decline. They're clever at hiding that fact as they bundle WhatsApp and Instagram users into their published user counts and both of those Apps are still growing so it hides the decline in Facebook itself. The charity now has a website again and it's brought in a substantial amount of money since relaunch.