It's a critical mass service. Once they own enough of your (target) customers' eyeballs, you can't ignore them. It's pretty simple:
1. Buy ads aka eyeballs aka popularity.
2. Subsidize early adopters via advertising to help them outperform competitors.
3. Gain critical mass.
4. Charge businesses to advertise on your platform. Make sure you pass along the costs for step 1.
You could do the same thing right now with a lot of industries as long as you have some type of carrot to dangle in front of the early adopters.
Imagine building a landscaping website / app where the extra value is that people who are having a hectic day can click a button and have someone show up and cut their grass. As long as you advertise enough early adopters would see a bunch of extra business, especially if they can pick and choose jobs to saturate their schedule.
Word would get around and every landscaping business in the city would want to get in on it, but not many of them would consider what's going to happen once you've monopolized the supply of customers. Spoiler alert! It's the same thing that happens in this article. You start charging businesses to advertise on your platform because you control their customer supply.