Facebook is used by everyone I know because it is convenient. It is easier to chat with people, message them, and post photos to them, on Facebook that it is to email individual people. e.g. if someone posts a photo we can all comment on it and discuss it together. With email this is not really possible.
The only reason a lot of my friends have email accounts is because they need one to sign up to online services like Facebook. They do not even check email. It is just a box with thousands of unread friend requests and spam messages. That is why it would be ridiculous to consider quitting Facebook for me. I would be cutting myself off. Obviously I would still keep in touch with friends via phone but we would engage a lot less.
In the end it depends on the ways people use it. If it's the only channel for communication, then you have to be in it.
Another question: do you really want to meet people who would exclude you from an event just because you don't have a Facebook account? Friends don't exclude friends...
Personally, I don't miss my account, but I can understand why others can.
I understand that leaving FB (or never using it) is not for everybody, but "ridiculous to even consider" goes way too far. I think actually it would be great for a lot more people to consider it, even if they end up deciding not to actually do it.