An acquintance of mine got his FB group going thanks to real life meetups. The online community was jump-started by the participants and grew organically from there to the point where 95% of members don't even know there used to be a meetup.

HN draws a lot of its user base from the fame of the Y Combinator VC firm, and a little bit from the occasional good thread being cross-posted elsewhere. So, a good recipe for creating a similar community is to attach yourself to an existing brand the way HN did. (As with HN, the results are going to be mixed.)