I worked at the IT help desk at my college when Facebook was first rolling out. We’d constantly get high school seniors calling up to try and get their college email address early, just so they could register with Facebook.
None of my friends at the time used it (or even MySpace) and I didn’t even have an account, so I found this very odd. The first time I realized it may have actually been popular was when a couple sorority girls came in and wanted me to make an account to friend me… not to actually be my friend, but because they had a contest on who could get the most friends. I did not make an account that day, and it told me everything I needed to know about how shallow the connections were. Those were in the glory days, 2004-2005, and it was already pretty shallow in certain circles. It only went downhill from there.
For me it was when Facebook sent the C&D order about the Whopper Sacrifice.