I feel bad saying this, as I've not a proper alternative in mind* - Facebook precludes anonymity of participants, which some may value enough to forgo participation.
*Can one participate in a Google group without a Google identity (G- mail, +, voice, talk)?
You could always create a throw-away FB account. But then of course it could be quite inconvenient to sign out of your usual account to sign in on this alternative one.
Update: There is now an online sign up to groups with workflowy, based on subject and current ability. You do not have to be signed up to Facebook to join a group, but do add an email address so that the group can contact you: https://workflowy.com/shared/cf1fd9ca-885f-c1b9-c2e8-e3a315f70138/
The recent Main article, searching for interest in LWers studying maths together, had many comments showing enthusiasm, but nothing really happened.
On an aside, I think that on LessWrong, we tend not to work together all that well. The wiki isn't kept bright and shiny, and most of the ideas we search for are in loose blog posts that often take a while to find. However, I think having a single place in which to work together on a specific topic, might encourage effect groups. Especially if it's in a place that you get fairly regular reminders from.
So, here's a Less Wrong Study Group Facebook Page: https://www.facebook.com/groups/131607983690959/
Rixie suggested that we could split into smaller groups, based on age. I was thinking perhaps ability. Maybe even a group leader. However, before sitting and pondering this for eternity (just until we have a perfect structure), perhaps we should 'just try it'.
So, who exactly do I think should join the group?
Well, if you're interested in learning maths, and think that being surrounded by LWers might enhance your learning, this group is intended for you. If you're interested in learning maths, but you think that reading a textbook on your own is daunting, or you've tried and had difficulty previously, then this group is intended for you.
Also, if you're interested in learning other LessWrongy subjects (perhaps some cognitive science, or more economics-y stuffs) then this group could do that. If ten people join who want a basic idea economics, then they can work together. This isn't specificly maths, it's whatever we make it.
Personally, when I read a textbook, there's often a paragraph describing a key idea, and the author's words fly right over my head. I've often thought the best thing for me, would to have someone else who I could talk through that bit with. Maybe he or she would see it easily. Maybe I'd see something they wouldn't get.
I also wouldn't worry about level of prior knowledge. Mainly, because mine is zilch :)
So, what are you waiting for?
(No seriously. Just try it.)
Edit: It is true that anonymity is difficult to preserve on Facebook. I am entirely unfamiliar with google, and I certainly would have to make that regular effort to check it there too. If you do wish to join but have issues with public knowledge, please PM me, and I'll keep in contact with you through email (or other if you prefer). I will discuss with you there how to best take part in a study group.