There's a mailing list? Have to admit I had completely missed that. I would be seriously tempted to move it front and center of the website, certainly once current news dies down. I don't know if you have stats but it would be interesting to look at how many people go beyond the front page.
In Terms of Twitter, one of the best ways to get followers is to follow people, if nothing else it's a prod for people to follow back. Encouraging people to interact and re-tweet is also useful.
Facebook wise, then PoP events seemed to work really well (in that I was invited to several through different sources). I notice that there is a Group but not a Page, I wonder if PoP needs a page, as far as I know stuff that happens in a group will not be seen outside of a group.
So I seem to be friends with one person on the Facebook Group (hello Denise), but if they post/like/comment on anything in the group then that won't get pushed onto their timeline, and there is no chance of seeing it. Whereas with a page there is at least a chance this will be seen by there friends who will hopefully also like, share etc.
As far as I know Facebook groups are much more about an existing group communicating internally which people are invited for or would search for. Facebook pages are designed to be more viral, and frictionless (I think)