Facebook has around 1B user profiles. They can be found by searching in Google. However, I don't think these profiles are linked from anywhere, so how could Google discover them?As far as I know, sitemaps are not enough for that (https://webmasters.stackexchange.com/a/5151), as all URLs should be crawlable anyway.I ask the question as I also have a site with user profiles and would like to make them discoverable.
Why Facebook profiles are Google-searchable?
google search;facebook;links
Contrary to what you suggest, profiles are linked to. Go to the Facebook homepage. At the bottom, you can see links to People, Pages, Places, Apps and more. If you click on People, you can see a full directory that's crawlable. As the disclaimer on that page notes, This only includes people who have Public Search Listings available on Facebook.