Yes, BRANDS can come from any category (ecom/manufacturer/info/comparison/forum/blog/social media), but to become a BRAND, you must transcend that category. Examples include
I would imagine brands would be names that co-occur near keywords in completely unrelated sites- possibly as shorthand in unrelated niches. So, you might see someone on a social site saying "I bought from amazon"; "I like drinking coke"; "I bought my Dell". Stuff where the brand name is sufficient to give meaning to a sentence, without context.
Now, once a brand transcends it's category, it no longer takes up it's categories' slot on page 1.
I would imagine Social Media and news would be the primary sources for Brand identification.