Where Facebook has made a virtue of these limits, MySpace has made a virtue of its lack of them. It now is so large—100 million-plus registered accounts—that it has almost come to be a proxy for the Internet itself. This is the problem with the social-media phenomenon. MySpace once enabled a remarkable social renaissance: Because of the site’s indefinable halo effect, you would answer e-mails you would normally never open, meet people you’d never suffer otherwise (“Bill O’Reilly” is one of my MySpace friends). It was, in fact, not unlike freshman year at college. But what’s remarkable soon becomes ordinary. MySpace remains cool—thanks to surprisingly deft stewardship by its new owner, News Corp.—but nothing is cool forever. And once the tantalizing pull of millions of people you could possibly be best friends with wears off, you’re left with some by now pretty ordinary functionality: blogging; instant messaging; photo, video, and audio uploads; networking tools. Thanks to the inexorable process of Web innovation, such stuff goes from “OMG” to “Whatever” in no time flat.

AOL followed a roughly analogous “walled garden” strategy until recently. The strategy died, in part because the limited offerings within the garden could not compete with the near-infinite wilderness beyond the picket fence. The social-media sites are touting their expertly tended, notably fecund, but still fenced-in offerings. And to be fair, they are free, unlike the old subscription-based AOL. But, as with the dial-ups, the distinctions in and among these offerings will become less interesting. Instant messaging, in yet another example of the Web’s buzz-to-blah dynamic, was once a unique and compelling reason to subscribe to AOL, not to mention hyped as a revolutionary application that would render e-mail fogeyish and vestigial. It is now a commodity function.

Few of the social networks have yet proved adept at truly linking people of like-minded interests, and many of the networks being started now, especially by entrepreneurs and corporations looking to grab their slice of 2.0 glory, tend to miss the reason the best sites work: They facilitate behavior that people already engage in. Networks that make intuitive linkages or networks that are built around more-organic associations (like the excellent new Saatchi Gallery social network for artists) may soon draw away users with more-sophisticated social-media palates.

But that’s just the start. The features associated with social networks are increasingly being tacked on to existing sites, and new ones are announced almost daily. Procter & Gamble has a social network now. So does Barack Obama. And someone will figure out how to network the networks, linking social-media sites and thus allowing iron-man social networkers to commingle their friends, blogs, images, and video feeds all in one place. (MiNGGL, Socialgrapes, and Wink are three new sites already trying to do this.) Individual sites could theoretically block out these epi-networks, but at their peril. Or a new craze could emerge that cannot (for technological reasons) or does not (for corporate reasons) sync with sites like MySpace, forcing users to divide their online selves in ways that become unsatisfying.