I went to Reddit for a while. It was novel at first having a whole community for any niche topic I could imagine, but I got sick of the same old memes and "discussion" dominating everything. They're so goddamn self-congratulating over there, too. Top comment on a given thread is usually somebody patting themselves on the back for having a totally controversial stance like "murder is bad" or "water is wet."
Reddit is utter trash and to call it an "improved forum" is kind of an insult to forums, because it has a completely different style and flow of discussion. It's a content aggregator, which is designed for the purpose of posting (other people's) funny images and (also other people's) funny comments for a spot on the top of the page. Which makes it very difficult to actually discuss anything, partly because of the terrible, echo-chambery community that style of website creates.
I very rarely see actual discussion when I go on it, it's all people looking for the first authoritative-looking poster they agree with and spamming it with upvotes and "THIS... SO MUCH THIS!!", because that's just how a content aggregator works. You upvote and save what you like and downvote and hide what you don't.
But to actually answer the OP, I don't think many people moved to other sites. It's more that people naturally rolled off as they came into things which made them have to prioritize life over posting, and because most internet newbies seem to find forums 'outdated' or inconvenient, they just go to Reddit or Discord for discussion instead. So little new blood comes in.
IMO, there was a slow drop in discussion (this is in multiple communities, not just Bay12) that began around the time non-human cities were removed from adventure mode due to glitches. I noticed people seemed to lose interest in discussing the game and just never come back to their communities. DF 2014 didn't generate the same hype as the game leaving 40d either. Hopefully the up and coming release and myth and magic can draw some more people in.