How many people are needed to maintain instagram, facebook, etc? Wikipedia isn't just a collection of static content. The page contents include static and dynamic content (via a lua scriptable set of templates), semantic data (wikidata), multimedia management (wikimedia commons), editing tools (WYSIWYG editing, with full support of the wiki markup and templating), global caching infrastructure, multiple datacenters for HA/DR, etc.

There's also the need to support the staff and volunteer developers, which includes wikimedia cloud services, git hosting, config management and orchestration, CI, community hosted tool/bot services, etc.

WMF has ~600 employees, and that's quite lean, for a service of their complexity.