I can't seem to find any evidence of a sitemap on Twitter, Facebook or Quora. Quora has THIS: http://www.quora.com/sitemap http://www.quora.com/sitemap I can'

I can't seem to find any evidence of a sitemap on Twitter, Facebook or Quora. Quora has THIS: http://www.quora.com/sitemap http://www.quora.com/sitemap I can't find any sort-of XML, JSON, or even TXT sitemap on any major site like that. They may have a special agreement with the search engines, or maybe their sitemaps only appear to search bot user agents/IP's. The other alternative could be that they simply DON'T HAVE A SITEMAP. Search bots might just be crawling them as normal, or with some sort-of link feed agreement with the companies, which lets them index as soon as a new link is created internally.