Just to mention one thing that isn't user facing (and therefore not so obvious): Social media companies dealing with user-generated content have to build their own enforcement mechanisms (abuse, copyright infringement, etc.), which is at least an order of magnitude harder than the user facing content engine itself.

For most startups that raise money, tech is hardly a big cost. Most of the money goes towards fueling growth through paid marketing and so on.

People and office space are traditionally two major contributors

They threw money at some big name early adopters.