[jlgbecom]

Let's assume there's total overlap, meaning everyone who has a non-FB account also has a FB account. Now let's assume all of those smaller, but substantially sized, networks started inter-communicating. Who would stay on Facebook? Why would you stay on the centralized network with 500 million people when you can spend all your time on the open network with 1.5 - 2 billion people?

As for active versus registrations, we can't be sure of that from anyone, but rest assured, a decentralized network has much greater potential than Facebook does. Especially in the rest of the world where Facebook isn't nearly as popular.

[Qz]

500 million vs 1.5 billion is a meaningless metric regarding which social network people will use. They will use the one their friends are using, and no one has near enough friends for those gigantic numbers to have any relevance.

[jlgbecom]

If it's such a useless metric, why is it always brought up as a clear example of Facebook's unmovable dominance?

[Qz]

It's a useless metric in regard to an individual person's decision to use facebook as opposed to something else. People don't care that facebook has 5 million users, they care that their friends use it. If all/most of their friends started using something else, they would too, regardless of whether that new network had 5 billion people or 5 hundred.