When is it right to split websites to sub-websites?

I'm wondering.

My question is a bit odd - StackExchange Vs. Amazon.

StackExchange is a mega website that revolves around Q&A. For some reason they chose to make a separate website for each topic. This approach seems to work.

On the other hand, Amazon is a mega website that revolves around selling things, almost anything. They chose to put it all in the same site under a huge categories side bar. This approach seems to work also.

I guess it has something to do with the character of the website. My question is, how do you know which way to go when you have a lot of categories?

Thanks.

Amazon is a mega website that revolves around selling things, almost anything. They chose to put it all in the same site under a huge categories side bar. This approach seems to work also.

Not everything. Amazon has some dedicated sites as well - http://wireless.amazon.com is one example. While it may share some likeness, it is a separate entity... found this out when I attempted to recycle a phone and then use the amazon credit to pay for a new phone (they wouldn't allow it).

When the user experience begins to suffer. Things continue to make sense contextually until there's too high of a concentration of things to deal with.

Amazon's philosophy is the world's market place, where you can get everything from "a to z." While it's a giant site, it revolves around a common theme with common values, serving a specific type of consumer (someone looking to buy um, anything really). One con of this is it's a very cost intensive operation to run this optimally, which isn't a problem if you can clear being in the red for as many years as Amazon has but still maintain investor interest.

StackExchange separates users by interests not because they won't co-mingle, but they likely feel that people benefit from participating in Q&As related to their interests. When you're consuming social information, it's a lot easier if it's curated (Reminds me of how much I hated Twitter until I started making lists). This is a much more cost effective situation as certain sites can scale up and down, and be dissolved or consolidated without impact on the majority of users.

The Area 51 stack exchange is kind of interesting. It's a single site where a few communities "test." Also, Quora is kind of like SE on a single site. Browse around and see how you feel in contrast over the two. Can't please all of the people all of the time, but you can address a market gap by filling in the niches, even if that niche is a giant site that sells everything. :-)

StackExchange websites are categorized too. For example, StackOverflow has several different categories (represented by tags, that is different languages etc.) The thing is, questions related to History, or English language have nothing to do with programming. So, it was wise idea to split the website. On the other hand, Amazon is all about online shopping. All the products have something in common: they are all products, and can be bought online. So Amazon categorized them. However, when Amazon came up with services (AWS), it split the website to aws.amazon.com, because the services are considerably different from products.

Well, the answer is, it all depends on the nature of the website.