> Facebook is the most used software in the world.
This is very obviously not true. For example, more people use PHP than use Facebook (supposing that there is at least one user of PHP who does not use Facebook, such as e.g. myself). I'll let you construct additional examples for yourself.
And if, btw, you argue that the users of Facebook don't "use" PHP, then I'll argue that they don't "use" Facebook in exactly the same way, since their web browser (or mobile client) is an intermediary.
