"Flashmobs were okay, but they had a couple of big downsides. First, they required you to go outside. Second, you had to, well, be in a flashmob.
Hashmobs solve both problems by transferring 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." 
"The canonical example (to date) of a hashmob emerged a few days ago around the hashtag #amazonfail. Amazon.com, as the result of a foul-up relating to its classification system for products, temporarily removed gay-and-lesbian-themed books from its sales rankings. Soon after the snafu, or, if you wish, FAIL!, came to light, a trickle of tweets labeled #amazonfail started to drip from the realtime faucet. The trickle promptly turned into a raging torrent - thousands of angry tweets an hour. The resulting hashmob spent a day or two pillorying Amazon and spinning various imaginary conspiracy theories, some of the more ridiculous of which fingered Amazon as a member of an anti-gay cabal run by Mormon elders. Because journalists have become some of the most avid Twitterers, the #amazonfail hashmob quickly gained a good bit of press coverage.
And then, as it became clear that the reaction had far exceeded its cause, the hashmob slowly dispersed. Tag it #amazonfailfail." 
"Though the #amazonfail event is important for several reasons, I can’t write about it dispassionately, because I was an enthusiastic participant in its use on Sunday. I was wrong, because I believed things that weren’t true. As bad as that was, though, far worse is the retrofitting of alternate rationales to continue to view Amazon with suspicion, rationales that would not have provoked the outrage we felt had they been all we were asked to react to in the first place ...
Whatever stupidities Amazon is guilty of, none of them are hanging offenses. The problems they have with labeling and handling contested categories is a problem with all categorization systems since the world began ... We know all that, but we’re no longer willing to cut Amazon any slack, because we don’t trust them, and we don’t trust them because we feel like they did something bad, even though we now know, intellectually, that they didn’t actually do the bad thing we’ve come to hate them for."