Instagram uses Facebook/Meta algorithms for serving ads, which includes behavior. I almost never post anything about diabetes in Instagram, but because many other sites (ie: reddit) use Facebook libraries for whatever reason they are using them, Facebook knows I'm diabetic, and I get served ads for diabetes products, bogus supplements that "cure" diabetes, health programs that use diabetes products for non-diabetes purposes (yes, this is a thing), and so on.
If a site uses a FB tracker library (which is going to be part of FB based auth), your usage is tracked and info based on that is used to serve ads across all Meta products.
Instagram uses Facebook/Meta algorithms for serving ads, which includes behavior. I almost never post anything about diabetes in Instagram, but because many other sites (ie: reddit) use Facebook libraries for whatever reason they are using them, Facebook knows I'm diabetic, and I get served ads for diabetes products, bogus supplements that "cure" diabetes, health programs that use diabetes products for non-diabetes purposes (yes, this is a thing), and so on.
If a site uses a FB tracker library (which is going to be part of FB based auth), your usage is tracked and info based on that is used to serve ads across all Meta products.