No, they mostly block specific content, and often because they don't approve of the content by policy – not because the receiver has objected: https://www.wire

No, they mostly block specific content, and often because they don't approve of the content by policy – not because the receiver has objected: https://www.wired.com/2010/11/facebook-link-blocking/ https://www.wired.com/2010/11/facebook-link-blocking/ https://444.hu/2016/11/14/facebook-is-reading-your-private-messages-and-blocks-you-if-it-doesnt-like-the-link-you-send-to-your-friends https://444.hu/2016/11/14/facebook-is-reading-your-private-m... (The intended receiver can be frustrated or confused that they couldn't receive the message their friend wanted to send.) And as you typically have to opt-in to messaging with someone new, they could wait for complaints rather than have content-based prior restraint. And, react to specific abuse with post-action blocking of the harasser, or quarantining-the-harasser as someone who'd need extra explicit acceptance before their messages are delivered.