An easy solution would be to offer a "no preview" flag. But this also doesn't make sense because presumably the person's email account (where the preview would

An easy solution would be to offer a "no preview" flag. But this also doesn't make sense because presumably the person's email account (where the preview would appear) is as private as their messenger. People who don't want to send info in email tend to send links to a secure portal, which they could do just as well in Messenger, preview or not.