It's clear to me that this was a spam/phishing message.
1. Comes from @facebookmail.com
2. Signed "The Facebook Team"
3. Includes only links to facebook.
4. In the footer, "Want to control which messages you receive from facebook..." facebook link.
--- Now, they obviously created a facebook event as a promotion and invited people on facebook to that event in order to generate those messages, so there is a bit of a grey area, but it's clearly a power.com email and there's no message of power.com other than:
Event: Bring 100 Friends and win 100 bucks
What: Reunion
Host: Power
Start Time: date
End Time: date
Where: Power
--- It's an unethical use of Facebook's systems at the very least, and spells out a pretty big hole (hopefully filled by now) that opens facebook users up to scams because the originator is obfuscated.