More importantly, in my opinion, is that Google can serve ads to people who are simply researching something, which is quite often significantly prior to the time at which they're ready to share with the world, if it's even something worth sharing.

Nearly everything is affected by a search for information. Not everything is appropriate or desirable to share with your friends/family. Facebook may make up for this slightly by having the ability to process private messages (whatsapp, facebook messenger, private groups) but I would conjecture that Google has them, perhaps fundamentally, beaten on the advertising front.