Those I understand, but I'm wondering about the "What's your perfect fall day" or "How old were you when you got your first job" questions that don't seem like they'd yield answers to security questions.
Secondly, this is almost like the email phishing paradox: to an educated user it seems like the number of people who respond with relevant information would be extremely low, but if the attempt costs you basically nothing and you get something useful 1% of the time, you're still winning.
"My perfect fall day is my memory of Aunt June when we lived in Connecticut in the 70s, before she passed away." In itself something like that doesn't seem useful, but there's a good amount of information in there if someone can correlate it with other details about your life.
Additionally, if your profile is public (the default still, I believe) is made available when you comment, I'd guess there'd be follow-up scraping going on to collect more details that are then used in conjunction with whatever your response was.
"How old were you when you got your first job" actually does seem like a good security question for many people. You're unlikely to forget it, after a while not many other people will know it, and it's a single number (whole number, most likely) so it's easy for a computer to parse (and hard for you to mess up by leaving out / adding too much detail).
Depending on what you respond to on social media, and depending on if you've got any accounts that use this as a security question, you might want to go back and force the account to use a new question ;)
And honestly I bet age of first job probably is a decent indicator of certain economic factors.