A site like Facebook would be impossible with just 10-15 people. Designing, implementing, and managing something on that scale is an insane amount of work. It probably takes a dozen ops/monitoring people just to keep the thing running.
a Website (httpd etc) + Web Application server (CMS/CMF) + Application Container (J2EE and all that) + RDBMS (Oracle, MySQL...)
And multiply that to development, QA, and production environments.
Yes, that way you need millions just in licenses and hosting to run even a lol-site and hundreds of employees to keep the beast working.