I have a photo sharing website, which provides very little textual content. Users can add tags to photos and a description, but it creates a lot of duplicate content, because most of the descriptions will be 'wow', 'lol', ... I don't think I should rely on users to build my SEO.I think it would be a great idea to write a blog, and use it to describe the best photos, start contests, explain themes, in short: create original content that search engines will love.Our website's main URL is like www.domain.com, and our new blog is hosted on blog.domain.com. From a SEO perspective, is it a good idea to keep the blog separate from the main site?  This has the advantage to leave the original site unchanged, but will it add any page rank to the www.domain.com? If the blog ranks well it will obviously pass some page rank to the original through links.What do you think is the best option from a SEO perspective? Include the blog in www.domain.com? Or leave it in blog.domain.com?
How to write a blog for SEO purpose
seo;blog;duplicate content
create original content that search engines will love.And therein lies the problem. Always, always, always create content for users, not search engines. This means useful content that humans will genuinely find compelling, interesting or derive value from. By what you've described, it sounds like that's what you're planning on, it's just a case of wording - thought I'd better make that crystal clear, anyway. :-)The Sub-directory vs folder debate is age-old. I recommend you read   some of these questions:Subdomains vs folders, for multi account/users applicationBetter SEO from sub-directory or sub-domain?Why should I use a subdomain structure over a directory structure?Moving from Subdirectory to Subdomain structureShould I choose sub-directories over sub-domains in this case?Personally, I would keep it all on one site, but that's just me.