Why does every social network have to be the only one? My opinion about the network effect.

It is obvious that every social media page tries to become the one and only one in the world. This looks like normal capitalism at first. But it's worse: Due to the so-called network effect those companies have to have a far bigger market share in order to make profit than conventional businesses.

My theory is that this has a lot to do with the fact that they have to sell advertising which is shown to their members. You can't make money with showing ads from every industry in the world to just a small number of the world's population. Therefore you have to have a large portion of the potential customers for every industry you can imagine in your network.