system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What makes facebook so valuable? Why would it be a 100 bn $ company?

Facebook is not making any products/ selling anything. Why would it be valuable?
gpt
They do have a product they sell, a very valuable one... You.

All those statuses, all those 'likes', all those location check-ins... That data isn't just there for your friends to see.  It is organized and categorized and analyzed and creates an unbelievable source of data for directed advertising.  You dump every detail of yourself onto Facebook which tells them exactly what to try and sell you.  The ads may be small and seem insignificant, but they are the digital equivalent of real estate on Manhattan Island.  Add to that the ads beyond the actual Facebook.com site itself.  The 'like' button you see on virtually every website now, ever wonder why you don't log in every time you use it?  A cookie based system is essentially tracking your every online movement while at the same time allowing advertisers to customize ads anywhere based on your FB info.  Not just Facebook either: Google, Yahoo, Foursquare, Twitter... All these 'free' products we take for granted as "Innovative communications tools for the 21st century!" are vast information fishing nets for well-paying corporate clients.

I always chuckle when the semi-annual "Facebook is going to start charging a fee!" outrage kicks up.  No they aren't.  They have zero need to ever extract a penny from us, more likely they will keep tacking on new free feature after another.  When you sell something you charge the fee to the buyer, and we aren't the customer... we're the product.