I believe the specific means of communication is not important here, it's the act of not using what everyone else in the group are using.
If most of his friends are using Facebook to organize events, he is the oddball that is hard to contact because he only uses e-mail.
This is equivalent to you stop using e-mail, or answering phone calls and insisting that you can be contacted only by fax.
Sure, your friends can probably send you a fax if they really want to, and your 2-3 closest friends may even do so, but you better believe most of your extended group of friends won't bother with that, and you're gonna miss out on a lot of invitations.
It's most likely exactly like that for him. Some people still keep in touch and go out of their way to invite him to events. Everybody else won't bother.
This means, that for a large segment of the population, opting out of Facebook is a huge sacrifice socially.
When the options are "use facebook" or "miss out on almost everything", for many people there is only one realistic option.