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.
@impiaaa the multidimensional matrix is existing in #hubzilla.
You can sort pll in different groups, like friend, family, friendly ppl, assholes I just want to send some love content into their streams..etc.
Depending on what you write you descide for whom that content is visable.
Also you can encryot messanges and encypt with a passwort, which might just you and secret friends know.
Like that you can have public and private in one post.
