[dudifordMann]

but in this case, the users, through the Power website, did give permission to receive, or send, the "snail mail".

From the article "Power users also authorized the software to send Facebook messages to other Facebook users for them"

So in your analogy. I created a business of sending snail mail to addresses I already had in my possession on my list of contacts. My contacts might not appreciate my snail mail, but I am not sending snail mail to the Facebook corporate Office -Or- if I am sending to the Facebook Corporate Office, then only though its mail routing department, which was set up to handle these very packages.

The C&D, I think, is equivalent to preventing contacts from communicating with each other, or, Postal Censorship[1], which I suppose is more a policy issue than a legal one. Which I suppose extends FB to be a governing body, which I guess leads us to CFAA...

[1]https://en.wikipedia.org/wiki/Postal_censorship