Good point! And It's not that hard to do. Shouldn't be much of a problem at all, actually. I'll have to use posts instead of threads, so it's not a perfect comparison, but this way it's really simple.

 Let's start with your post. It's #4,485,530 on VGChartz as a whole. That's on the 26th on March.

Number 4 million was on the 11th of April 2011. That means going from 4 million to 4.5 million will have taken pretty much exactly a year.

Number 3.5 million was on the 23rd of June 2010. Meaning that it took 9 and a half month from 3.5 to 4 million.

Number 3 million was on the 28th December 2009. Thus, it took half a year from 3 to 4 million.

Number 2.5 million 18th of August 2009. Meaning that it took 5 and a half month from 2.5 to 4 million.

Number 2 million was on the 8th of April 2009, meaning it took 4 and a half month from 2 million to 2.5 million.

Number 1.5 million was on the 1st of December 2009. So a bit over 5 months from 1.5 to 2 million.

Number 1 million was on the 16th July 2008. So 6 and a half month from 1 to 1.5.

Number 0.5 million was on the 11th February 2008, meaning it took 5 months from 0.5 to 1 million.

In other words, there's been quite a drop just lately in the VGChartz popularity, but not a drop that coincides with the sales forum popularity. Most of the sales forum drop happens before 2010, while virtually all of the overall popularity happens after. 

You definitely had a point though. Personally, I didn't at all expect VGChartz forum popularity to be as low as half of what it was during its peak.  

 Edit: To put that a bit better, here's a table form. Note that it's very rough, and the point is to give an indication, not for the number to be exact.

I'd like to point out that ioi stole 36.739 posts in December '09.
http://gamrconnect.vgchartz.com/post.php?id=2963261
-missing-
http://gamrconnect.vgchartz.com/post.php?id=3000000

The drop in overall forum popularity unsurprisingly coincides with the introduction of the worst version of VGC. It happened in the middle of 2010. VGC IM got removed. Wall posts were limited to about 250 characters with no ability to make paragraphs. PMs didn't allow you to use paragraphs, everything became a wall of text. The plus sign wouldn't show up in forum posts. A lot of stuff that was easy to do previously had to be learned to be done differently.

In other words, the means of communication between members were greatly reduced and/or limited which meant that a lot of members left the site. That's why the amount of time it took for 500k posts suddenly doubled.

As for the sales forum specifically, 2008 was the peak year because the generation was still fresh and a lot of possible scenarios could be discussed, but from 2009 onwards most trends were set in stone and a lot of historic sales analysisses had already been done. A decline was bound to happen eventually. If you look at 2011, the second half was lower than the first half, even though the holiday season should spur more discussion. But the seventh generation is old, so nobody really cares anymore. Almost everything that could have been said has been said.

Of course, VGC taking graphs and other stuff away from us didn't help, but looking at the number of threads, the sales forum would have remained stagnant at best.