The details are hard to find but the last reported number of employees is about 1,200.
If you wanted to create the same site, you would have to nearly match that size and have about 100 programmers to pull it off along with others curating content.
The same looks to be true with the other two sites. These are companies with over one thousand employees each. I guess you wanted to read where a lone programmer could make such a site.
I have been wanting to create a website like the following example but would like to know what is the potential cost involved in such a project. How many developers do I need and how long will it take? Any tip or advice would be appreciated.