I claimed, following the logic of my last two posts here, that in some cases, the Internet is working better as an attention aggregator for matching than any physical mechanism could do. I got some push back that if the goal is to stimulate social connections between geographically proximate actors, even the success stories on the Internet, like meetup.com and craigslist are not anywhere near as good as the traditional mechanisms like political yard signs (and presumably block parties, though they didn't mention that). I think that may depend on how physically proximate one requires the connections to be.
Below, I try to separate out the push-pull dimension from the targeting dimension. I think the targeting of messages to their intended audiences is the key thing that the Internet can potentially do better than any off-line communication effort. And if we think there are some positive social capital externalities from social connections with "neighbors" who live a mile or two away, as well as those who live down the block, then some targeting will be valuable, and the continued development of matching services on-line is a hopeful sign for social capital in America.
Dimension 1: Push vs. pull. In "push" delivery, the sender exercises some control over the content or timing of message delivery; with "pull" the receiver exercises more control. This is really a continuum rather than a dichotomous variable, despite the usual punditry. You see a lawn sign or a bumper sticker or a pop-up Internet ad as you pass by, even if you were not looking for messages on the topic. That's push.
While push may be attractive to message senders, there are both privacy and information overload issues with it. The privacy problem occurs when the recipient is annoyed by the message's contents or its timing. Information overload occurs when there are a lot of messages pushed. As more messages vie for attention, the success of any individual message in attracting attention declines. Push can be effective only if there aren't very many different messages being pushed (there can be many repeated messages so long as there are only a few kinds, as information conveyed visually by quantities of repeated messages, as when the green political signs outnumber the blue ones on a block).
Dimension 2: Targeting. There are two dimensions of this (related to the ideas of type I and type II errors in binary classification). What percentage of the people who are exposed to the message are interested in it? What percentage of the people who would be interested in the message are exposed to it? In addition to the obvious communications efficiency advantages of more targeted messages, there are potential privacy advantages to the sender. Someone may be far more willing to reveal their interest in knitting (or radical libertarianism) only to people who also reveal that interest. Consider the staunchly pro-war neighborhood where the few opponents might like to find each other without exposing themselves to the ridicule (or worse) of most of their neighbors.
Note that targeting and push are not necessarily mutually exclusive, even in the off-line world. Consider abortion protests outside of clinics, where the intended audience of the message are congregated. Still, it is often difficult to target a specific audience effectively with billboards, bumper stickers, and other geographically constrained message channels.
Usually, to get better targeting, pull modes work better. Putting a for-sale sign in your car window can't hurt, but to increase the percentage of car shoppers who might see your message, it's wise to place a classified ad in the paper. To reach an even larger percentage of potential buyers, you can try eBay Motors, but if you have a cheaper car that's not worth shipping to distant buyers, then you'd only want to reach local buyers and listing on eBay would have poor precision in message targeting (a small percentage of the people seeing the listing would be interested in your car). Similarly, lawn signs may be good for alerting everyone that you support Howard Dean for President, but they're not a very good way to find the small percentage of people in your town who would want to attend a fundraiser or help you leaflet at the state fair.
Clearly, there are more opportunities for targeting in communications that are brokered through the Internet. Meetup.com and the kind of Internet-based geo-matching may be more promising for finding the people who want to leaflet at the state fair. Conversely, there are fewer opportunities for effective push on-line (spam and pop-up ads notwithstanding), in part because people have fewer spare attention cycles while on-line. While in a traffic jam, people are often happy to be amused by a bumper sticker on the car in front. Not while reading email or surfing the web.