One of the easiest ways is Meetup.com. If you don't find a community that works, you can create your own. Twitter is helpful for finding people and their new Circles feature looks interesting. I don't use Facebook much these days, but have joined several groups in the past. It's a lot of work, but I also once (pre-Twitter) built my own website with a forum and posted articles and tutorials that got pretty good search engine traffic and pulled like-minded people to my site.