If they didn't have the incentive to get a monopoly we would have kept a more decentralized web.

Did you miss Myspace and Friendster? Orkut had a market, I think. The reason that Facebook succeeded later is because it had a good API and invited people to write apps for it (mostly surveys and games, remember Zynga?), tracked its users across the internet, and sold them in targeted slices to advertisers. It was Friendster as a commercial platform (like Salesforce was CRM as a commercial platform.) It was never any better than Myspace, and gradually got much worse.

I have no idea what you think is or was so great about WhatsApp. It was successful largely because it had no overhead and infinite throughput, being created by very talented people using Erlang. The reason people use it is because they always have and their friends do, and for a lot of countries with expensive SMS it provided messaging (and became so integrated into business that people weren't allowed to live without it.)

This is my argument about why these things were successful; I think that "they were great" isn't a credible one. They simply grabbed a large chunk of already proven markets quickly, but oriented themselves toward corporate needs to leverage that into total control.