The question is, how does that turn out? The theory says people should keep creating dating products to make Match buy, because it costs less to do than they have to pay. That doesn't mean it happens instantaneously. It doesn't mean they can't buy some of them. But it's an evolutionary process. They buy the ones they can buy, until one shows up they can't. Maybe because there are too many of them. Maybe one gets too big too fast. Maybe the owners are some stubborn purists who refuse to sell. It hasn't happened yet doesn't mean it won't.
Apparently Facebook is getting into the dating game. They already have the network effect and Match doesn't have the money to buy them. Facebook sucks, but now where's your monopoly?
And what's to stop FB dating from sucking. So the final solution is to replace one monopoly with an even bigger one? That's just talking out of both sides of the mouth.
Match makes shitty products optimized for revenue. Good dating products optimized for users have low revenue and valuation. Match acquires virtually all of them and keeps its monopoly alive and thriving.
There is no point arguing with reality.