Facebook did not kill my class reunion. A total lack of interest killed the 10th, and the 20th was cancelled because they decided to start everything at 5 pm on a Friday afternoon in October, and everything would be done by 3 pm the next day.
Because everyone still lives in town, right? Well, all the important people (ie, the ones organizing it) do, at any rate.
I'm not hard to find on FB. I use my real name, and, AFAIK, my contact information is still current. (My high school still manages to find me a couple of times a year in order to hit me up for money.) But the FB group is only about 50% of the graduating class, if that - and no one seems to be looking for the missing people (like me).
It's not just my class. Back when my sister was coming up on her 10th, the people in charge sent out an email saking "Where are they now?" Sister looked up most of them over the course of a day or so, confirmed that it was correct, and sent it off. A month or so later, the same "Where are they now?" email went out, looking for the same people that my sister had helped locate. IIRC, she did not attend her reunion.