If one day the social network is no more you probably will lose all your content (maybe they allow you to download it in weird formats that you have to learn how to export to another platform).
Most social networks are powered by proprietary software and it’s not clear what they do with your data / who has access to it
Privacy
Most social networks are from USA and must respect the laws of that country. Since it’s a country that do not respect human rights (e.g. freedom, privacy) I rather not associate myself with it.