It looks like I will be helping a small firm with their online social media presence a bit. I have been blogging since around 2004/2005 (and involved in all kinds of usenet/listserv communities since long before that.)
I like this presentation on the "power of artifacts." Kim mentions that online conversation always produces an artifact.(i.e. a "thing" - a blog post, a discussion thread) and he discusses how powerful this can be.
Kim also does not discuss what I consider to be THE big issue with building online community - Trust. On the internet no one knows you're a dog. You could be a trolling sockpuppet - a weasel in dog's clothing.
Kathy Sierra death threats --very violent/sexual and graphically illustrated threats were made to a prominent professional blogger. I remember the blogosphere discussion. Some bloggers fervently dissed Sierra's fearful reaction, and supported the "right to free speech" of the anonymous poster.
The troll or sockpuppet can hit and run or quietly insinuate -- engage in guerrilla warfare. The anonymous troll wields asymmetric destructive power. He knows a lot about you. You don't know anything about him.
Online communities are groovy but also risky.