So far, flash mobs are just artistic events, performances that are fun to participate in. But this is just the beginning of experimentation with a new social possibility: coordinated activity among strangers without an institutional framework or mass media to coordinate it. For example, someone recently posted a pointer to a suggestion that distributed but coordinated political protests could be more effective than converging for a rally. It's not completely thought through yet, but an intriguing idea. In any case, it would be foolish to write this phenomenon off as purely whimsical.
This is a really exciting time we live in, if new social configurations get you excited. I view smart mobs (and flash mobs in particular) as just one more form of experimentation with new forms of social organizing that have been developing over the past decade or two. Recommender systems for movies and messages, eBay's reputation system, and geocaching all fit the bill as well. The common theme is that they involve action and interaction among strangers. Friendships sometimes develop out of the activity but they are not pre-requisite. People are developing trust and coordinating activity in large networks, without becoming friends or even acquaintances.
We might think of established friendships and institutions as a "just-in-case" form of social organization. Relationships are in place so that action can be taken as the need arises. We might think of these new ways of interacting in large networks as "just-in-time" social organization. Some relationships are in place in advance of activity, but they may be few and weak.
If neither friendship nor instutitional membership is a pre-requisite for coordinated activity among strangers, what are the pre-requisites? I've coined the phrase "impersonal social capital" to refer to whatever those enablers are. It's some combination of networks of acquaintances, generalized trust, assurance through reputation or other accountability mechanisms, and a big dose of technology to reduce communication and coordination costs. I took a first stab at trying to sort this stuff out in a paper titled, "Beyond Bowling Together: SocioTechnical Capital" a few years ago. It may be time to take a closer look again, now that there are more examples to draw from.