Yes, but we can make some educated guesses.

For instance, what "good" has Facebook brought us so far? After we have determined this, is it not reasonable to extrapolate a little into the future? Also, entertainment is not unimportant, but everyone would agree it certainly does not belong anywhere near the top of the list, yet I see a lot of effort going in that direction lately, also on HN. It seems out of balance if you ask me.

I am not a Facebook fan, but it was successful at connecting communications around the world and increasing overall adoption of the the internet.

I truly mean it, it's an unanswerable question, educated guesses or not. Too subjective.

For instance... is it better for Earth to have 1 Billion people on it, or is it better to have 25 Billion? If a "tragic" disaster drastically reduced the population levels for 100 years, but also allowed our species to evolve at a more sustainable pace in the long-term, was the disaster good or bad?

The disaster would be good for the species, but bad for individuals. The species is more important than any number of individuals, so such a disaster would be of net benefit.

Speak for yourself. I'm certain there are people working on hard problems on HN.