I would say a good billion of Facebook users are either highly inactive, actually inactivated but Facebook is still counting them, scammers and porn bots, and dead people. Many people made an account in like 2011 and promptly forgot their password and never logged in again I'm sure. And lord knows Facebook isn't honest about anything.
I'm surprised Twitter is so comparatively small but it's also a weird site in that you literally talk into a void, at least Facebook you know your uncle is reading your post or whatever.
Ok my bad. But what do they count as activity would also be a good question. Posting? Just opening the app? What constitutes activity to Satan Zuckerberg? I wouldn't be surprised if just accidentally opening the app even if you're never posting or reading counts.
Since 2010 i've made three Facebook accounts, deleted one and never use the other two. Active users would be a more useful metric.
It's been commented on before that, for its size, Twitter has a disproportionate influence not just on the internet but even on politics. This is still the case today because no good alternatives have emerged for the mainstream yet.