For users who don't have a set username through Facebook, there is no way to email them. Facebook does not allow messages to be sent via email using a user's ID - only their set username.
However, with a bit of quick research regarding these IDs, it looks like Facebook has completely discontinued this e-mail service. As multiple articles turned up, it sounds like the email-to-Facebook-message service ended in 2014, though Facebook would still forward emails to email@example.com to that user's set primary email address (so if someone emailed firstname.lastname@example.org, the message would appear in that user's primary email address, i.e. myFakeEmail@gmail.com). That message-forwarding service appears to have ended in ~May 2016.
If you still want to test this for yourself, you can; but even if it somehow still works, Facebook has never supported email-to-ID, so that unfortunately won't be possible.