What do you think? Does bitcointalk posts number has decreased due to bad market condition and investors leaving for the mean time?
That could probably one reason. The campaigns that are paying out in crap coins now has coins that has significantly lower prices due to the market crash, hence most campaigns might not be worth it right now for people hence the decrease in activity(assuming that activity here in bitcointalk actually decreased since the crash like you said).
The bigger factor though, in my opinion, is the not-so-recent 1 merit requirement for Jr members, and also the recent endless bans on people who committed plagiarism. The forums was far more active before that new rule. The forums are definitely a lot better though; activity doesn't necessarily mean better. I'd prefer low activity but high quality rather than the opposite.
By the way, I think this topic is more fitting on the Meta subforum[1]. It'd probably be better if you moved the topic to that subforum.
[1]
https://bitcointalk.org/index.php?board=24.0