Twitter doesn't need 3,500 employees, Facebook doesn't need 17,000 and Google doesn't need 72,000. They could all fire 50%-90% of their workforce and the product wouldn't be materially affected for the vast majority of users. The reason they have this many is because they can have this many. Their success has given food to the cancerous growth.
You get this situation whenever a company isn't operated by a shareholder who get's dividends.
When the person who runs the company knows that every dollar they don't spend is money in their pocket, people start to actually care about expenses and only focusing on what is important.