The data you post isn’t more private like other commenters have mentioned.

On the other hand, lemmy and other open source social media platforms won’t collect behavioural data like other privately owned social networks.

Instagram for instance will track exactly how much time you spend looking at each post to determine your interests and predict which ads you are most likely to click on. Others than that they will also run experiments of changing some features and track your engagement with them.

So in essence, they collect more data on stuff that you don’t explicitly share.