You can figure such things out roughly.
https://web.archive.org/web/20080310002752/http://www.gbatemp.net/ should be able to be set by year (or by month if you fancy a bit more granular), and the forums here have stats you can figure out (as well as maybe check on the archive site).
Total users will tend to grow as old ones don't get purged but you can check numbers per year there to see rate of growth, and maybe samples of active user count (though that varies by day, and we have the occasional viral thread or flood if a rumour happens or hack releases).
The above and sorting by start date in various sections* will give you an idea of new thread count and active users. New post count can be harder but still doable if you are really bored.
*there have been new ones made and things shuffled around (reviews became a site section all of its own rather than a section of the forums, the creation/splitting of the computer section is another, as the hack methods crystallise (few flash carts on the 3ds, more about those custom firmwares) then so do forum sections, depending upon how far back you want to go then
https://gbatemp.net/forums/books-music-tv-movies.191/ spun off from general off topic, and the politics section appeared as well, the xbox 360 section got merged into one, you can probably also witness the slow death of the user submitted news section) so be careful of using raw numbers there or use section totals.
Don't know if the notable members stuff (
https://gbatemp.net/members/ ) and equivalents for older forum versions will be in archived but worth a try as you might get an idea of some of the people active and not.
The fortunes of Nintendo consoles have an effect on traffic -- nobody was here for the Wii U scene really, and it was lean times after the DS died (the wii had long since died new game wise, not to mention new games/scene releases stopped being posted here) or was otherwise a solved issue and before the 3ds all kicked off in earnest (nobody anywhere cared about the DSi, least of all when most of anything was the cycloDS).
