This last paragraph is more purely speculative than the rest of this post, but I suspect that many of the members of this community, at least those who spend a lot of time here, participating in all aspects of the culture, might not have strong cultural roots outside of here. This is based partly on the types of person I suspect are attracted to xkcd, and partly on what drives someone to spend significant amounts of time here. Again, I mean no offense, nor do I mean to over-generalize. Instead, the point I am trying to get around to, is that one of the more beautiful aspects of XKCD and this comic in particular, to me, is that it HAS (whether Randall explicitly intended this or not) created both a community and a culture, that provides a place for people to belong, to enjoy and to PARTICIPATE in the evolution of the culture.
I certainly didn't mean to imply that there weren't a lot of different cultural backgrounds represented here, and I agree totally that it adds to the wonder of a place like this, that so many different people can exist in relative harmony. I will try to explain further, not in any attempt to counter any of your points, all of which are well taken and I agree with, but because I enjoy expressing my thoughts on these matters. I think XKCD has always had a strong appeal to the type of people that might be classified as nerds/geeks. No, not everyone reading is a geek, and certainly not all geeks meet the normal stereotypes, but much of the comedy on the XKCD strip requires knowledge of math, physics, computer science etc. And certainly, much of our discussions on here are of a "geeky" nature. We exult in using small shifts in star patterns and planetary placements to work out a suggested time for the comic strip, we "discuss" whether a slightly changed curlicue on a letter in a strange language places it among category A or category B. We gleefully argue about whether an object looks more like a telescope or a theodolite, etc. For quite a while, a strong geek/nerd subculture has existed in modern society as a whole. I'm not even sure that "subculture" is the right phrase, since it implies, to me at least, a sub-group existing within a culture. In this case, at least since the internet exploded in popularity, "trans-subculture" might be a better description, since it is really a subculture that exists in many cultures, and connects across them. This is, in essence, what you were celebrating in your post, not specifically referring to nerds/geeks, but in a more general sense, that the culture here pulls people from many different cultures together into one place. And, it isn't quite the same melting pot effect as, say here in the USA, where our culture has been formed strongly by the influence of other cultures, mostly through the effects of immigration. The culture here on this thread doesn't seem to me to pull strongly from other cultures, but rather seems to be composed of many cultural viewpoints, but is really it's own thing entirely, growing from nothing and with the strongest influences (again, in my opinion only) being from the geek/nerd trans-subculture, rather than from any of the individual cultures of the members.
All of this is a long-winded way to get around to the point. Subcultures often form when groups of people feel somewhat separated from the their own cultures for whatever reason. The "mad scientist" didn't become the "cool nerd" until computer science became such an important and necessary part of modern society. Nowadays, a geek is just as likely to be a weightlifting, athletic jock, or MMA fighter as anything else (well, maybe not JUST as likely, but still not uncommon), but it wasn't long ago that there was more of a stigma associated with it, and jocks vs. nerds movies seem to still be quite popular. I think I've rambled a bit here, but I'm finally getting to the conclusion. This thread provides a place of enjoyment for many, but for some it also provides a strong sense of belonging. I believe (acknowledging that this is without any attempt at formal proof) that there is a correlation between the amount of time spent on this thread, and the level to which the thread provides a sense of belonging and not just a place of enjoyment. Commensurate with this, is that a need for a sense of belonging here may reflect a reduced sense of belonging elsewhere. Let me emphasize that this is one of the things that I think is beautiful about what has been created and continues to be created here. A place for people to go, and feel like they belong and that other people appreciate them, who might not get that elsewhere as much as they want. It's "Cheers" for a whole group of people who have likely been following XKCD for a long time. If you do, in fact, spend 4 or 6 or 8 hours a day on here AND have a full time job AND have significant other activities with other cultural groups, then you definitely need less sleep than I do and I am equally happy that this place provides a community and culture for you as well.