Let's say you have a technology product that does something pretty trivial, but has non-zero value. Let's say you do something that's worth about 30c per day of utility to each user. If you give this value to 10 million users, you're creating about $3 million of value every day --- or $1 billion of value a year.
That value doesn't seem nearly as real as the equivalent value deployed to a smaller group of people. But that's just because 10 million people is impossible to visualise. The value is still totally real though, despite your lack of intuition for it.
If you believe Facebook delivers little total value, then you must believe its value to individual users is almost totally nil, simply because of the enormous number of people who use the service daily.
Even if you don't like Facebook much, can you really argue that it's delivering _nothing_ to its average user? It's not delivering _any_ value to them at all?
Facebook has 500 million users, and 250 million daily unique logins. If logging on to Facebook delivers even a single cent of value, the total value delivered by each Facebook engineer is already going to be staggering.
1c is an enormous underestimate of what people actually get from the service, but let's run with this. At 1c per login, it's delivering $25 million per day. It has 2000 employees, so each employee is delivering an average of $12.5k per day. I'd actually put the value-per-login at over $1, which would take the average contribution to over $1 million per day.
People would be wise to think carefully before questioning the way markets are allocating capital and drawing in talent. There are certainly cases where the capital allocation is inefficient, but there are also cases where your mental heuristics give you blind-spots.