Is "Le Web 08" organized solely via Upcoming.org and Facebook, or do they have their own registration service? Many people might have seen the announcement on Upcoming, but might not have added their avatar to the listing.
I think random Google searches are also not the most common way to attract people to such conferences.
Exactly. Random Google searches aren't the way to attract people to such conferences. But an invitation from a friend with similar interests also attending the conference is! facebook 1, Google 0.
If you think about it, what is Google the way to do? Anything involving people, and facebook wins. Flickr is Google-searchable. Facebook Photos is not. But facebook Photos wins, at least among young adults.
If that's what people want... Email also isn't searchable, yet people managed to inform each other about conferences via email for years.
No doubt Facebook is a certain power on the internet. I still cling to the "social networks are like nightclubs" theory, though, and expect people to move on. I simply can't imagine Facebook dominating everything, because I personally don't like it.
I think random Google searches are also not the most common way to attract people to such conferences.