I became internet bestfriends with someone and she took not just mine but some of my bestfriends photos that we posted on social media and she used them but creepily she printed our photos out and she also posted pictures of me on all of her social media accounts. Would it be illegal to use our photos without us knowing?
it’s almost certainly legal
The licence you grant when you post on social media is usually incredibly broad (different companies have different licences so I’m speaking generally). They usually allow anyone to use them for anything they want.
For example, here on Stack Exchange you agree to an irrevokable CC BY-SA licence which allows Stack Exchange to share it and adapt it providing that they attribute you and allow everyone else to do the same.