Besides, what of interest could he possibly be sent?
"Dear Mark, I am writing because I have a great business opportunity for you... ...
...
...
... and that's why you should buy out my revenue-positive minimum viable product so we can pivot towards disrupting even greater markets in the social space.
Sincerely, Every startup in the world"
Yes. I'm sure it's a great idea and only the highest quality of spam will be sent his way. Please forgive my cynicism ;)
As for a point of order, this wouldn't be considered spam anyway:
A message is Spam only if it is both Unsolicited and Bulk.
Unsolicited Email is normal email (examples: first contact enquiries, job enquiries, sales enquiries)
Bulk Email is normal email (examples: subscriber newsletters, customer communications, discussion lists)
Have a nice day.
And god knows that a business guy like Zuck would never do anything that's not nice.
Steve Jobs' one sentence (if even that) replies to people who sent him emails are a fun part of his legacy at Apple. Maybe Zuck is trying to strengthen his image as czar of Facebook, powerful but still social.
Edit: after thinking about it, I guess the fact you're paying to message someone would impress the recipient, thus enticing them to reply.