Any business that wants to accept incoming email over the internet will need an SMTP server, so that facebook has one isn't unexpected. It is an incoming server, so won't accept facebook source addresses.
It appears to be fairly standard, and wants standard headers:
# telnet 184.108.40.206 25
Connected to 220.127.116.11.
Escape character is '^]'.
220 smtpin.mx.facebook.com ESMTP
250-smtpin.mx.facebook.com says EHLO to xx.xx.xx.xx:57628
MAIL FROM: firstname.lastname@example.org
RCPT TO: email@example.com
250 MAIL FROM accepted
250 RCPT TO accepted
354 continue. finished with "\r\n.\r\n"
Date: Thu, 16 Jan 2013 19:01:32 +1100
From: Paul <firstname.lastname@example.org>
User-Agent: Mozilla/5.0 (X11; U; Linux i686; en-AU; rv:18.104.22.168) Gecko/20090122 Lightning/0.9 Thunderbird/22.214.171.124
To: Paul <email@example.com>
Subject: Subject Line
250 OK A8/71-30754-8D667F05
This email came through to my facebook account without issue. Note that everyone on facebook has a facebook.com email address, and SMTP is how email is delivered on the internet, so the requirements for incoming mail on the facebook SMTP servers is much the same as any other - if they weren't, it wouldn't work.