They say that duplicate or "false" accounts make up 10% of their total numbers. They also are effectively banned in China and Russia to my understanding. So they are claiming that more than 1/4 of the whole world (even the undeveloped parts) use their website once a month even accounting for these false users.

Facebook is blocked in China, not Russia. However, there are plenty of Chinese MAUs when factoring VPNs and those who travel abroad. Basically, you can subtract the total Chinese population from the world population and assume you've properly discounted the global population to account for the few other countries where it's banned and factored in those who travel or connect via VPN (Iran, North Korea, etc).

I'm curious, what's the use case for having multiple Facebook accounts? Presumably you're talking about actual user accounts, and not Pages.

My wife's friends/family use them as a form of silo'ing different groups of people. I haven't used FB in years but there used to not really be a good way to do that, at least. That would be my guess. For example, my wife is friends with her own sister 3 times as 3 slightly varying names.

I won't get into my mom's use case. I think she mainly uses them to friend with and snoop on people. Weird lady.

I use one for my personal use. Another for video calls for my kid. Another for snooping on girls I slept with when I was single. A fourth for apps that want third party login.