[ryan_j_naughton]

Most of the time when two sided markets gain market power through a dominant position, they still aren't a monopoly. They are an oligopoly employing game theoric positions that are worse than perfect competition but far from a natural monopoly.

For example, even with Amazon there is Target and Walmart selling online and doing much more similar delivery times these days.

The network effect of a two sided market definitely is a barrier to entry, but there is so much capital out there that other large firms or firms with access to insane amounts of capital will also build a 2 sided market and create a network effect. It's why there is Shopify, eBay, Etsy, etc. All more like an oligopoly than a monopoly.

In contrast, true natural monopolies need barriers to entry that are so high that it just doesn't make sense or isn't even possible for another company to do it even if they have insane capital. The electricity grid prior to "deregulation" and the forcing of a more competitive market was a prime example and the grid being governed/run by an independent service operator.

[barry-cotter]

Exactly! If the threat of competition holds prices below the profit maximizing levels for a monopolist you may have a local monopoly but it’s not a natural monopoly.

[QuesnayJr]

A pure monopoly is a textbook notion, like a frictionless plane. In practice, there's more or less market power, and thus more or less of a role for government intervention to correct it.