As far as I'm concerned, the only reason people like Facebook is because the email listserv is not very discoverable. Sometimes, you want to be included in the stream by default but don't want to participate right away. An email chain is too easy to exclude people.
I use Facebook only insomuch as I have friends who use it and send me messages through it. Most of my "social" interactions through the internet are through email now, with groups of friends setup on lists so we can tune in and out as we want. It's easier to search than Facebook's past, too.