The facebook integration is worth 0 to me. I want to collaborate on text, not accidentally click my avatar and be taken to my profile page.
I think the facebook integration is a waste of your development time. If you keep it, keep it only for the easy population of pic+email+username. Also, get rid of login if the pad is truly open to everyone. I'll keep using one of those other etherpad instances, since there's not much facebook integration gives me over a stock instance.
Speaking as just one guy, "log in with $service" is a bigger pain than "Sign up and log in". Sometimes I don't have $service, or I have it but don't like using it, or I have it but I don't want whatever new random thing I'm trying out to have hooks into it. (Anyone else seen in their Twitter feed a bunch of app noise followed by an apology?)
I dropped GMail about a year and a half ago. If any services I was using required it to log in, I would have had to drop them, too. I think it's mostly because I know a lot of programmers, but I do know a large number of Facebook "conscientious objectors" (myself included) who can't log in to things that require Facebook.
It's really nice to have services like this self-contained (as long as it makes sense; Twitpic, for example, would be less useful if it had no hooks into Twitter). I can pick the service up and if I don't like it, put it back down. And if I drop some account elsewhere or just change accounts, nothing depends on me having it to log in.
Just putting myself on the list as yet another person who would be trying this out right now, but it requires a Facebook login, so I'm writing this comment instead. I don't use Facebook or Twitter, and I would not be pleased with creating a permanent link between my Google account, which is important to me, to some little application that I may forget about quickly.
Unfortunately, they aren't doing anything interesting.
With all respect, hackpad has a fraction of the features of the other etherpad forks (including the original etherpad). That's not necessarily a bad thing, but nor does it give me a reason to use it over say piratepad.
I'd consider an OpenID login; I already have one on myopenid.com for Stack Overflow. But I don't like the idea of logging in to a service with any kind of identifiable "persona" where I don't have strong control over the connection. I don't trust Facebook, my Google account is too important to pollute it with random site logins, while I'm afraid a Twitter login would somehow end up giving permission to Twitter DM me, which I don't want.
I have no facebook account, I don't want one and I certainly don't create one to use your version of etherpad. No offense, just think about it...