There are only a few dominant social networks because consumer markets are prone to winner-take-all effects. There are multiple reasons for this.

First, consumers want stuff for free or cheap which drives consumer companies to expand aggressively so that they can amortize their fixed costs over many such users.
Second, consumers want to conform with other consumers so inherent virality in products gets built-in. Once, a minimum threshold of consumers adopt a product, this word-of-mouth virality ensures the product becomes an obvious choice for the remainder of consumers left in the market. If you have Uber that all your friends are using and it has all the drivers in your city, why would you try something else? ...