Finally, I should address the spread of exclusive content. First, a member of the original community shares the content in another community, not realizing the meaning does not carry over. You can see this on the various Holla Forums facebook and twitter appendages. Secondly, as a result of the first case members of the other community go to the original community for more/new content to share further.
This is where the reddit mindset is important: as reddit by design rewards sharing popular content - regardless if it is original. Users who, either appreciated 4chan content, or saw it was popular went to 4chan looking for similar content. As these users weren't familiar with the culture and so never took the time to understand the meaning of the joke. As a result they reap content with reckless abandon, creating a new, self-perpetuating, copycat culture. You can see this behavior on r/4chan.
In conclusion: imageboards benefit from their culture of creation, their insularity and their exclusivity. As users are promoted to create a lot of content is made, their insularity provides authenticity to what is created and make it appealing to members of the community and the exclusivity of it all provides an air of mystery. However, this definitely does not make them the "driving force of internet culture" - a cursory glance at just youtube can inform you about that much. What would be more accurate is calling imageboards the driving force of internet pop culture: imageboards used to influence the more out-there and esoteric content of the internet. Stuff that you'd share, just because it was different from the grain even if you had no idea what made it stand out.
After all: imageboards are the "armpit of the internet".
This is all roundabout and reductionist (I haven't even touched on user psychology, material background and many other, relevant factors). Regardless, I hope I have given you a general idea of the history and culture of imageboards. To prevent misconceptions from forming, I have written a few notes:
The things I sum up about reddit are not an endorsement: all of those things are valid criticism of some sub-reddits, but these criticisms were not the original point of contention that made 4chan users hate reddit.
I am not claiming reddit is exclusively an aggregator, nor that imageboards are exclusively originator. There are communities within reddit that are quite prolific, while there are boards on 8ch that barely create original content. However, each website has a dominant culture and I think my analysis holds up well for these.
Tumblr is very diverse: it is not exclusive to its notorious, psuedo-feminist community.
This is a broad intersection of the history of 4chan as a content generator. Chances are I made small mistakes here and there. However, I'm confident most of this will hold up, as I am a content creator myself with many, many, many years of experience with producing content for imageboards.
There is also the issue of later generations of users building myths around certain cultural concepts. The best example is "normalfag" and is always good to ruffle some feathers: "normalfag" was not, originally, used as "normie" is used now. "normalfag" was originally used as an insult for people who pretended they were 'normal', or people who were obsessed with appearing normal. (Which shows how insular 4chan was: it was inconceivable for an actual 'normal' person to show up, instead it were weirdos pretending they were normal. It fits right alongside "there are no girls on the internet [4chan]") The fabrication that it is a surrogate for "normie" or 'normal' is definitely created later.
One can also think of 'newfag' and how the meaning of that term has transformed throughout the years.
Can't stress enough that, to users of imageboards, any given imageboard is more than just a plateau to make things. It is a community in the full sense of the word.