Ask HN: What are the key SEO considerations when building a social network?

A lot has been said and written about how to do SEO and content strategy for B2B startups and e-commerce. However, what are the key things to cover when building a social network such as Facebook or LinkedIn etc?

The primary thing we want to rank for are the Google search queries for people's names, i.e. If someone is searching for "Gajus Kuizinas", we are competing with LinkedIn.

We have ideas about off-page SEO. However, given that we already have so many unique pages (multiple pages for every user), is there anything that is a must to do?, e.g. Does a sitemap make sense for a social network that already has hundreds of thousands of users?