FB's solution to unsolicited messages and spam was basically to make messages from non-friends totally invisible. I wouldn't really call it a dark pattern, but they have broken the ability to send messages to anyone who is not your friend. It's pretty much a spam filter with with one binary parameter. It's just lazy.
Interesting point, I wonder if they can apply some logic into it, like if the sender is male and he's written to many female non-friends, give him a higher "probably junk" score. But if he's gotten good response rates his message is probably worth delivering.
FB probably already has data to know how much of a pervert someone is... if they linger on those beach pictures for too long, for example.
