Facebook rightly puts the burden of compliance on the developers, not the users. Spam scores would just lead to user experience and customer service nightmares. "Why does it say I'm spamming?! I'm just playing a game! Fix it!"

I think messaging on Facebook could be an excellent ecosystem for a lot of things. A spam score would work or allowing people who make shitty requests to be blocked from doing so on the request.

Users know exactly how annoying it is to receive such invitations having received them themselves.