But as Evan Williams told me, “Celebrity twitterers are really outliers, even though they get a lot of attention.” Williams believes that the service is best understood as a system that swiftly routes messages, composed on a variety of devices, to the people who have elected to receive them, in the media they prefer.

Twitter’s elegance lies in its extreme simplicity. Pownce is more complex. As with Twitter, one can send messages to friends or groups of friends as well as to the service’s general community. (Unlike Twitter’s messages, Pownce’s cannot be sent to mobile phones.) But you can also send your friends links, invitations to events, photos, pieces of music, or videos. In addition, you can finely discriminate which group or subgroup of friends will receive a particular post. It is this combination of private messaging and file sharing that makes Pownce seem so richly functional. Such features are more often found on fully formed social networks like Facebook; but Pownce retains much of the intimacy and directness of Twitter.

Pownce was cofounded by Kevin Rose, the cofounder and chief architect of the hugely popular news aggregation site Digg and the cofounder of Revision3, an online video production and hosting company that shoots ­Diggnation, a weekly news show that Rose cohosts. Much of the excitement that attended the launch of Pownce last June derived from Rose’s reputation for creating new-media companies that hypnotize their youthful audiences into cultish devotion. Pownce seemed especially cool because Rose decided that only those with invitations would be permitted to test the new site.

Most of the other microblogging services combine some features of both Twitter and Pownce. Jaiku, for instance, works with cell phones, as Twitter does, but like Pownce, it is more friendly to pictures and videos. A few have novel variations on the basic themes: Kyte grandly claims that it allows “anybody to create their own interactive TV channel on their Website, blog, social network, or mobile phone”–a kind of microblogging that bypasses the written medium altogether.

Critics of microblogging argue that the services are not sustainable businesses, because they merely float upon the speculative bubble of venture capital investment in Web 2.0 companies. More nastily, they complain that almost all microblog posts are stupefyingly banal.

Bruce Sterling, the journalist and science fiction writer (whose latest short story can be found on page 69), crisply articulated the latter argument when he wrote to me, “Using Twitter for literate communications is about as likely as firing up a CB radio and hearing some guy recite the Iliad.” The private-equity markets best express the first argument: while the micro­blogging sites could not exist without venture capital, the sums invested in them have been relatively small. (Twitter, for instance, reportedly received about $5 million from Union Square Ventures and other investors, a paltry figure for a company whose importance has been so hyperbolized by the media, bloggers, and its users.)