Interesting! That's what I expected but I think the vertical matters quite a bit. My old startup (http://getpressi.com/) that had more of a tech crowd focus definitely had more people signing up with email while the new one (https://makersalley.com/) has a more general population that tends to like using Facebook.
At this point our total users is in the hundreds so who knows how it'll turn up.
Yep - it's definitely just a data point.
There are obviously hundreds of variables in play here from the content of the site to the fundamental design that may be influencing this.
So really, just a data point:)