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.