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Please link to that post. I don't know what number they are pulling, but that'd make them bigger than facebook, which I don't think is the case. It's likely that this number is the total log ins, not unique log ins. So not 1.8 billion accounts, but maybe 300 million logging in 6 times a month or something of that sort.

I think its unique, and I also believe Facebook has over 2 billion monthly active users which still make it number one.
Here's the source of my screenshot https://www.theverge.com/2018/5/3/17317274/youtube-1-8-billion-logged-in-monthly-users-brandcast-2018

Please let me know if I missed or misunderstood something here.

interesting - yeah, that puts them very close to facebook.