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.
I think this has at least some truth to it. Keep in mind that there is an IRC channel, though I am not sure how much it gets used. I would endorse the creation of subreddit for rationalists (which would encourage all manner of offtopic discussions) as opposed to a subreddit for discussion of rationality.
Two corrections: I think you mean 'bonding' in the title, and I think you mean 'price' not 'prise' in the 2nd to last paragraph.
I used to frequent the IRC, and still go there occasionally, but in my experience it suffers from a bad case of Well Kept Gardens Die By Pacifism. There are no moderators, no gate keeping process, etc. and as a result the quality of discussion usually sucks. Many people there are not even interested in rationality at all.
I'd NOT recommend it, nor linking to it prominently on the site.