[gaius]

Another thing FB effectively fixed, app invites notwithstanding, was spam.

Probably lots of kids these days don't even know this, but there was once a time when your (probably only) email address was supplied by your university or employer. If two people moved, it was easy for them to lose contact with each other until a "chance" encounter (e.g. seeing them post on Usenet or whatever). Facebook solved that problem by making everyone's address book self-updating.

[chc]

That was already fixed by the time Facebook came around. Hotmail was offering addresses that weren't tied to your uni or employer in 1996.