Hashmobs [transfer] the flashmob concept into a purely realtime environment. A hashmob is a virtual mob that exists entirely within the Twitter realtime stream. It derives its name not from any kind of illicit pipeweed but from the "hashtags" that are commonly used to categorize tweets. Hashtags take the form of a hash sign, ie, #, in front of a word or word-portmanteau, eg, #obama or #obamadog. The members of a hashmob gather, virtually, around a particular hashtag by labeling each of their tweets with said hashtag and then following the resulting hashtag tweet stream. Hashmobbers don't have to subject themselves to the weather, and they don't actually have to be in proximity to any other physical being. A hashmob is a purely avatarian mob, though it is every bit as prone to the rapid cultivation of mass hysteria as a nonavatarian mob.
I suppose what looks like a mob and what looks like effective grassroots organization and protect depends on where your bread is buttered.
Everybody together now: Who Cares?
(Sorry, Alan, but that's my general response to all Twitter-related blog posts, especially ones relating to the equally irrelevant phenomenon of the "flash mob".)