I agree with many of the points here, but I want to add some of my personal thoughts, mainly about Discord. I find Discord preferable to FC because of its fast pace and constant conversation. It is much easier to make a quick remark on Discord than write out a whole paragraph for FC. So it makes sense that people (especially younger people) will prefer this more fast-paced platform. Additionally, while it is nice to have informational guides on FC and it's true that it's easier to store these guides on FC than Discord, I prefer to get my ant info from Discord. When I was a beginner, I could simply ask a question in the appropriate channel and often get multiple answers. Sometimes I would be provided multiple points of view and different experiences which were extremally helpful. Though, on some occasions my questions would be ignored or covered up by others so it is not without faults. Another thing about Discord is that it allows for much closer connections than FC due to it being a more fast paced constant conversation. I personally have made many a friends with other ant-keepers through Discord. Nowadays I don't use Discord or FC much due to time constraints and lack of interest. But I think they are both great platforms that each have their own place in ant-keeping. And while FC does seem to be on a small decline, I am sure that it will continue on well enough as long as the hobby continues to grow and it continues to be moderated.
P.S. If you haven't tried Discord, I suggest that you give it a shot. It's definitely not for everyone but you might be surprised by how useful the platform can be, especially for niche hobbies like ant-keeping.