I find myself talking more and more with people about community gardening these days, and there are some thing I'm slowly learning to explain. Here's one common explanation, or at least the current version of it - does this make sense to you?
I'm glad you're interested in learning more about community stuff! You asked how you could build online communities - I'd like to try rephrasing that and asking first what kind of community you'd like to grow - and after we figure that out, then we can sit down and talk about what kind of tools and practices can help with that, and what makes sense to do online. The internet is not a magic wand that somehow makes community happen; it's a tool that can take away some of the physical-distance barriers to participation.
One way of thinking about this is imagining that teleporters have been invented, and that they're instantaneous and free. Yay! You can do a lot of things you couldn't do before. But the existence and availability of teleporters will not magically solve all community-growing problems, and if you were to say to people "we can use teleporters to work on our project!" they will nod sagely, continue with their lives, and nothing will have changed.
People still have to make the time to work together, make things together, form relationships, have conversations. The time and attention you need to devote to those things doesn't change; teleporters are basically removing the commute-time cost. You still need to make sure people are working on the same thing in the same place at the same time, or if their schedules don't work out to have the same times open, agree on a place to leave notes at and a way to keep track of work asynchronously... you still need to help them find out about what others are working on, encourage them to work together, report their progress. All that stuff.
Now, we don't have teleporters. But we do have chat rooms, webcams, mailing lists, and the like - tons of tools we can choose from to connect two or more people. Those tools are not as high-bandwidth as in-person communications, but they'll suffice, and they do have some advantages - for instance, automatic logging - over in-person meetings. (And for the record, I prefer the fewest number of the simplest tools possible, so it becomes a nearly-invisible help rather than a Shiny Technology hindrance or distraction. I'm sort of a Luddite that way.) Choosing the right tool becomes a lot easier when you know what you want that tool to do, though.
So. Don't think about the internet or particular technologies for a while. Imagine teleporters. What would you like your community to look like?