Now THAT would be a fun class. Sorry for the wall of text, but I enjoy discourse analysis and social dynamics:
I forgot to add that the only time I've really seen a single "topic" thread (meaning not one like WAYWT or something that is regularly updated with new material), go 700-800+ posts is when a flame war develops.
With almost no exception, I don't think a single-topic thread can be informative past 300 posts. Beyond sheer material exhaustion is also a paradox: any single topic on a public "general interest" forum won't have enough "expert" knowledge to keep it going without tiresome repetition. Conversely, assuming such experts all taking part in the same thread, it becomes hard to follow.
Looking at the science of social networks... my guess why this is the case is even though we are online, I don't think we can go beyond the cognitive social limits evolution has given us; I don't think we can really pay attention to 40 people all sharing a lot of information at once. At best, we focus on 4-5 and skim the rest (similar to what we'd do in a large room... focus on a few and mostly ignore the others). We simply aren't equipped to handle more than that, whether online or IRL. On any given SF thread, there are probably not more than 4-5 informative posters worth listening to, so in general we don't exceed those limits. As well, those 4-5 generally are given an obvious leeway/deference by others, so that even when there are 50 or more on a thread, the "leaders" (like any social group) quickly become obvious.
*This is perhaps one problem with CE; everybody thinks they are the leader on a thread and so with 30 people, 29 of them all think they are the only one contributing information (so it becomes an obnoxious, almost unreadable echo-chamber)*
Having been on many fora since 2000 or so... IMHO what is required to keep a forum going is not what you'd expect; it is this wide range of non-expert poster that makes it fun. I've also been on those "private" forums made up of only experts, and it generally burns out quickly with a lot of catty bitching. So, you really can't have your cake and eat it, too.
Even then, there is a brief lifespan to a topical thread, with peak fitness at 30-100 posts. Less than that, it's finding its legs... more than that and the real meat has been shared, after which the vultures swoop in.
SF is sort of like the RL of menswear fora; somehow it hasn't diluted its "brand" even given a number of different quality levels (after all, for every Manton/Purple Label there is a mensimageconsultant/lauren Ralph lauren.)