Quite a while anybody could send information to Facebook users because Facebook implemented the open standard of Email.
Sure, I could send emails to your facebook account, but if I wanted to see any of your social media content, I would have to start a Facebook account and access it via Facebook’s walled garden. If I want to use Google+ and you use Facebook.…
It’s as though you had to use a Verizon phone to have a conversation with other Verizon users, and you couldn’t use your Verizon phone to contact people who use AT&T. The outcome is inevitably a monopoly due to Metcalf’s law.
I don’t want everybody to send me messages in an unfiltered way.
Your social network client could still have filters, but the filter would be something you control, and it wouldn’t be as arbitrary as “you may only friend-request other facebook users, and only other facebook users may friend-request you.”
As far as breaking up Facebook, I don’t see a reason why they should have Instagram and WhatsApp but the core Facebook service can’t be easily broken up.
Start with an open standard for friend requests; i.e. Google+ must accept friend requests from Facebook and vice versa. Any new startup would be able to create their own social networking client, capable of sending, accepting, and displaying friend requests, media shares, private messages, wall posts, etc. This would create a much better, more competitive system with vastly more consumer surplus.
It would only happen in areas of dense human habitation, which already wrecks the ecosystem. No net harm.