There is no doubt a situation of frustration occuring on all sides.
Agreed.
Can I ask you now that you have posted what you feel is the reason for signifcant decline in sciforums membership?
I have not been active for some time, so I'm not sure that I am really qualified to answer, but I'll give it a shot.
Firstly, I am not sure how we are defining "significant decline", nor what period of time is being considered. From my POV, I seem to see a lot of new members that have recently arrived but only seem to stay for a few posts - usually less than 100. Concurrently, I have noticed that a lot of old timers seem to be absent or at least less active. My immediate take would be that the reasons are as many and varied as the number of members under consideration. Take Cosmic for example, his farewell thread mentioned certain personal, individual reasons for leaving if IIRC.
To properly answer the question I think one would need to identify a sampling of members that have left or are no longer active and see if enough information could be garnered to identify a common pattern. To my knowledge this has not been done, as I said, I'm not even sure the underlying question has been properly defined.
So we are left with "gut" feelings. As has been previously pointed out, Social Media has probably taken its tole, but I think you may be looking for an answer endemic to Sci itself. It would seem to me that the influx of new members are qualitatively less prepared to offer content coherent and interesting enough to pique the attention of the long time members, and I'm quite sure many of the old-timers get tired of rehashing the same old tired questions that can be answered by a simple Google or Wiki search. So maybe part of the question is why we are not attracting the calibre of newcomers needed to replace natural attrition, thereby contributing to the perceived
decline in membership.
Many here long for "the Golden Age" when things were fresh and new, or at least are remembered in that fashion. Free wheeling "intelligent" discussions with more tolerance for pseudo-flame wars and a general atmosphere of fun and frolic. Now and then a member from that era will resurface, witness the recent return of Bebelina, for example. She may be a good source of insight on this topic.
As to the moderation strategy here, I would agree that the forum has "matured" over the years and some of the rambunctiousness has been curbed. This, of course, is only to be expected. Perhaps even inevitable. Is this a good thing? Probably depends on your POV. For all we know, the site is more profitable to the owners under the current paradigm than it was previously. We peons are not privy to this sort of information, so to an extent we operate in a vacuum.
In summary, I do not believe that there is a singular factor that could be isolated and quantified that would satisfactorily explain the "decline". Again, that's just my guess...