Welcome to WebmasterWorld Guest from 188.8.131.52
Some of these communities are over 10 years old and have hundreds of thousands of threads. Until the end of 2012, we've always done quite well in search engines.
With forums, it's no secret that Google wants the world to sit in its own forum (google plus) every day, all day. Devaluing other forums in their serps represents one strategy for doing so. Individuals looking for information via search, will no longer be sent to forum sites that compete with google's fourm site.
I like search results that direct me to forum pages with multiple posts from the same thread (such as Webmaster World pages), but I hate it when Google sends me to a forum where I see only one post (even if it's just a sentence or two) and have to click my way backward or forward to read the entire thread. Is it possible that such "thin content" forum pages have made Google view forums less favorably in general?
Have you checked Google Trends to review what the trajectory of your topic is, flat, up or trending down?
Maybe we're not talking about the same sprint forum?
On my main site, I'm contemplating moving the forums to a subdomain and curious if anyone had any experience good or bad after making a move.