I doubt it's in numerical order. I joined in October of '08 and my number is 1856. So I can't see that many joining since then. Just aren't enough posters to justify that high a number. Many of them MIGHT have been "Bots" that got deleted but still retain the membership number.

But to give you an accurate answer would probably require you to ask someone like Renee, Dan, Tom, ColdOne or the Webmaster.