Content. If you have great content, people will naturally want to share it.
I'm not 100% sure, but I'm fairly sure that the big website companies out there do not worry much (if any) about SEO.
As long as you cover best practices, and have great content.. you should be off to a great start. There is a lot more that will help boost rankings such as server location, site speed, SSL, mobile friendliness, how your content is displayed, proper use of coding structure, sitemaps.. but in the end it all comes down to content.
Of course, nobody knows for sure, since only a handful of people in the world know the exact search engine ranking formula.