Facebook: Friends are always mutual, and require a request. You can choose which posts are publicly available, and which are for friends.
Twitter: All posts are public, unless you have set your account to private, and then followers will need to request.
Mastodon: Private or public accounts. Every post has one of four privacy levels, which can't be changed later, and there is a multidimensional matrix that describes what posts can be seen by what users from where at what time.

This matrix AFAIK is not documented anywhere in full, and is hard for users, even long-time ones, to learn and understand.