Yes: you don't own the messages. It is a private internet that FB owns. That's why
I
don't like it.
The internet is meant to be decentralized. I use Mail Service X, you use Y, maybe one of us runs our own mail server. Doesn't matter. A common protocol makes it work. If my mail server dies or your service goes offline, any emails we've sent still live on the remaining server.
If we want private communications, we can encrypt our messages. If we want to use email as an API, we can. We can receive and parse and archive to our hearts' content.
When FB owns both ends, owns the servers, controls the features, and keeps the data, we communicate at their pleasure. If they want to scrape for political keywords or decline to deliver messages about banana bread, they can. If they want to delete your messages, they can. If they want to forward your messages to advertisers, they can.
"So what?" people say. "Gmail scans your messages and shows ads." True. But if I'm ever uncomfortable, I can switch to any email provider I want, or set up my own server.
If John and I both use Gmail, I can leave Gmail and still stay in touch with him. But if John and I both use Facebook, I can't leave Facebook without losing the ability to message him. If everyone I know is like John, leaving Facebook is voluntary exile, and nobody wants to do it first.
The internet is meant to be decentralized. For many people, FB is taking over the internet.
That's why I don't like FB.