People search for lost relatives. I know that this one is true because I have found myself searching for a long lost female cousin of mine from time to time. I haven't found her yet. I used to get along with her really well when we were kids and early teens. Then that branch of the family moved down south and except for some letters I never heard directly from her again. I know a bit about how her life went from some other relatives, but at this time no one knows exactly where she is. Anyway, people search for lost relatives this way. It could be an adopted child searching for their biological parents, the biological parents searching for that child they had to give up, or perhaps siblings who were separated when young. There are probably many more types of lost relative searches.
People search for past love interests and old partners. Sometimes people search for old flames, ex lovers, or maybe to check up on divorced or separated spouses.
People search for old friends and comrades. Yes, people do searches for that old high school or college friend. Perhaps a reunion is coming up and they want to touch base or see if that person plans to attend. Maybe they are on the planning committee and need to hunt down people from that graduated class year in order to send out invitations. They might be looking for donations to the alumni fund.
People do background checks. Perhaps a careful lady is going to date someone and wants to make sure that in this age of creative theft that the other person is who they say that they are. Maybe an employer has an internal person who does their background checks instead of dealing with an outside agency. Today the Internet has turned many old regime paid services into "self service" jobs.
So, there you have it. I just outlined several reasons that people search out other people online. There are probably many many other good reasons why they do this, and these combined reasons would explain the intense popularity of online people search.
The bottom line in all of this is that people search is huge and growing by leaps and bounds.