Facebook Has several different rules for different search engines.

But if you go to the end of file. You will find this one rule.

User-agent: * Disallow: /

The "User-agent: *" means this section applies to all robots. The "Disallow: /" tells the robot that it should not visit any pages on the site.

site:facebook.com returns About 3,520,000,000 results in google.

Then my question is why is facebook.com indexed in google and other search engines?