I have noticed that given how much I identify as a rationalist, how much I have in common with the community here, how important I consider it, etc. I have surprisingly little instant in group identification with community members compared to other online communities. There seem to be an aspect of social involvement that LW does bad at. And there is one thing lacking that to me seems the obvious first suspect; the lack of of-topic unstructured chatter.
What I do when I feel that I identify with some continuity online is in fact not usually the thing the community is ABOUT. Instead, it's the things that grow out of the sides; forum games, members art projects, photo share threads, fanworks. I can speculate on why this happens this is so, but it dosn't seem very useful at the moment, I'm not highly confident on any specific theory, and most will probably find it fairly obvious anyway.
LW, however, has no real room for this. Even in the discussion section, things that are not reasonably on topic will be punished with negative karma. Now, this is obviously needed, but one must still recognize there IS a prise to being so structured and focused on a single goal when humans naturally tend not to be. Look for third options.
Now, I have a specific solution in mind, but I'm going to hold of on proposing it and see if you come up with something better before I post my idea.
EDIT: My suggestion has now been added in the comments, please check it out.
The comparison is a group whose aim is not socialising in itself, but something else. In that sense I think it does compare. Particularly the Wikipedia of 2004-2005, when meeting other Wikipedians just started taking off. Though meeting up in 2011 is still going great guns and proving very productive.
I see from your comment that you were thinking entirely in terms of on-site possibilities. Have you been to either a Wikipedia or LessWrong meetup? Having been to both, I think the social aspects of a not inherently social site invoked by them are highly comparable.
Except LW IS an inherently social site, only restricted to a specific subject matter.