I do. Primarily because Facebook caters to users with low technical ability and includes baked in privacy settings that are notoriously ambiguous and difficult to understand.
Facebook actively promotes an anti-privacy styled online presence.
Those with the skills to build a website in '97, along with the requisite metatags to allow for the type of indexing you describe can reasonably be expected to understand what they are doing.
Not so with Facebook.
Adding content to a webpage is a pro-active move. Vanilla Facebook with no security tweaks or other expert knowledge (by my understanding) leaves users wide open through no action on the part of that user.
Did anyone opt-in to being included in Graph Search results? If they knew what it was, would anyone do this?
It's an interesting question that you pose and I think there is definitely more to it but the answer above is from the top of my head. I will mull on it some more and add should I come up with anything further.
I built a website in or around '97 on geocities. You talk about metatags being required for indexing but I think you forget that the web was young once. Google had a voracious appetite to index and they still do.
The barrier to entry for someone to build a website in '97 wasn't much higher than it is now. Things just look prettier these days.