Mark Zuckerberg's Meta started Threads three years ago. It now says it's up to 500 million monthly active users. I wonder how many people actually know they're posting there.Basak Gurbuz Derman/Getty ImagesMeta's Threads says it has 500 million monthly active users. A lot of those people are cross-posting from Facebook or Instagram.Some of those cross-posters don't seem to realize they're doing it.One classic — and sad — example is a woman posti…