Stories. So you think about all the people you've known, the interactions you had, and some folks naturally wonder "I wonder what ever happened to ..." Its even a cliche in movies where during the final credits little pop-ups appear over the characters and say something funny about what they ended up doing after the events of the movie took place. So if you happen to run into someone from the "old days" there is often a period of "catching up."
Facebook scratches that itch in spades. Sometimes in a quite creepy way. But it is insanely entertaining to lots and lots and lots of people.
I probably met 2 to 3 thousand people in my life and I honestly couldn't remember their names. Part of the magic of life is remembering those experiences, moving forward, and creating more experiences for you to live. Knowing that I crossed paths with interesting people is enough for me. If I really wanted to keep in contact with all the people I knew, I would have kept up. A simple email would suffice in most cases.
The fact is that we all change and we all grow apart at some point. Your statement makes me wonder how easy it is to slide into that unhealthy territory where you live in your past, and that is my philosophical issue with it.
So what if I missed out on a high school crush? Chances are she is married and has a kid by now. The real chance is that I see her now and realize she is not that pretty, not that smart, or has the same exact personality that I remember, but now it grates into me. There are other people and experiences hanging around the corner that are more than happy to see the future with me, no matter how brief.
I couldn't care less what my friends in HS had for lunch at the nightclub last night. I could take great photos and make it look like I live in paradise.
Anyway, the point I took issue with is the idea that Facebook is a necessity, and your statement does not prove the case in point at all. Why is it a necessity to look back and wonder? Why is it a need to connect with a large group of strangers? Why is it a need to see what someone you haven't seen for 15 years does for a living? Fine, check because you are curious, but once you check 5 times, you are inching closer to the line of obsession and refusal to let go.
Facebook scratches that itch in spades. Sometimes in a quite creepy way. But it is insanely entertaining to lots and lots and lots of people.