What is the "social glue" that binds a community together?

This is the topic of our class discussion for the next 2 weeks. We're thinking about both online and offline communities and what makes them good experiences. As I think about communites that I have been a part of, I appreciate the memories of when I felt like I was valued by that community. I like to feel useful and valuable. This seems like a basic life need, and I think Maslow's Need Hierarchy mentions it. I also appreciate the feeling of being "connected"...being part of an interconnected web. In other words, knowing that there are people who know me well and whom I know well. I like the feeling of being in a group of people who share similar life-interests and life-goals. It seems like humor and having fun are another important part of community, as are sharing meals, and sharing life's joys and sorrows. Some of these things can be done online, others are not yet possible online. The point for me is that online communities extend the possibilities for people with similar interests to come together. I liked how this week's article pointed out that the net is just another tool like the jet plane, phone, printing press, or writing; and as such, extends the reach and possibility for community. So what are the things that bind us together into a community (either online or in person)? What is this "social glue"? What do you think? Leave a comment!