“There’s a thin line between clever and stupid,” went the faux maxim from Spinal Tap, yet it seems to apply pretty well to Web startups. One of the most notoriously over-the-line is the social-networking site Tagged.com, which New York Attorney General Andrew Cuomo says he plans to sue for false advertising, deceptive business, and identity theft.

What makes the case timely is that many of Tagged.com’s practices, like mass emails and data mining, have become commonplace among social media sites. But Tagged’s aggressive combination of these digital promotions set it apart from its competitors, Cuomo charges. According to the attorney general’s press release, “Consumers who visited Tagged were tricked into providing the company with access to their personal email contacts, which the company then used to send millions of promotional emails.” He added: “This very virulent form of spam is the online equivalent of breaking into a home, stealing address books and sending phony mail to all of an individual’s personal contacts.”

In a blog post, Tagged co-founder Greg Tseng stated, “We are dismayed that Cuomo’s office, which has shown itself to be fairly well-versed in the Internet, would issue an inaccurate and inflammatory accusation.” Tseng denied that Tagged was trying to trick anyone and attributed the email outbreak to a faulty registration process.

Cuomo’s actions follow a raft of consumer complaints about the site, whose functionality seems designed more for perpetuating itself than for offering utility to users. Time called it the world’s most annoying Web site. Yet it has managed to become the third-largest social network on the Web, claiming 80 million registered users.

Tagged is one of the myriad MySpace also-rans that never achieved mainstream traction, but built a niche following. Tagged has a disproportionate share of African-American and Hispanic users, most in their 30s. Eight million users logged onto the site in June; that figure has more than doubled in the last year.

Where did this juggernaut come from? Tagged formed as a college side project of its co-founders, Harvard graduates Tseng and Johann Schleier-Smith. After snagging a $7 million investment from the Mayfield Fund in December 2005, both men took a leave of absence from their Ph.D. studies at Stanford to run the site full time.

Tagged works more or less like any other comparable site. Users create a profile, post photographs, and send messages to contacts. What differentiates it from better-known rivals, like Facebook, is the emphasis on meeting strangers rather than keeping in touch with existing friends.

A feature called MeetMe allows users to set specifications for gender, age, or location and view the results one by one, responding to the question “Are You Interested?” Two people who respond “yes” to each other’s profiles become friends on the network. If this sounds like a jazzed-up version of a personals ad, that’s because it is.

One feature that speaks to the flirtatious character of the network is the site’s namesake, Tags, which are small user-generated graphic icons that members can select from a central library and post on one another’s profiles, like dog tags.

Since other networks, notably Facebook, already have third-party applications that permit similar image sharing, it’s unclear what unique value Tagged offers members once they join and build their contact list.

Earlier this year, the company started using Tags as a recruiting device. New users were encouraged to let Tagged scan their email address books and send invitations to the social network, with a few Tags attached, to their email contacts. Recipients would then have to join the network to view the images. While members had the option to select individual emails from their address book, many of them clicked to let Tagged send the recruitment message to all their contacts.

“The good news,” says Tseng in a blog post, “is that more than 3 million people joined Tagged with this new process.” The bad news is that many members didn’t mean to send the recruitment message to everyone and claimed they’d never seen an option to say “no.”

New York State law considers that omission of key information, or even insufficiently conspicuous disclosures, as “false advertising.” Tagged is circulating a series of screen images showing a clear opt-in screen that users had to click through before the emails went out; Cuomo’s office says the screen shots are doctored (an accusation Tagged denies).

More complicated are the charges surrounding the email, where the graphic Tags icons were described as “photos,” leading recipients to assume that friends had sent them snapshots, as is common on photo-sharing sites like Flickr. Moreover, the emails appeared to be sent by the friend (and Tagged member) directly and often included the member’s profile photo in the message body.

Cuomo’s office charges Tagged with false advertising in its description of Tags as photographs, and identity theft in its use of members’ names and images in the recruitment emails. Granted, as Tseng points out, it is “common practice of virtually all Web sites that offer an ‘invite or forward to a friend’ option” to use “the name and email address of the person who has chosen to forward content as the ‘From’ line.” This includes news Web sites as well as social networks.

But Tagged’s tactics went further. If an email recipient chose not to register with Tagged to view the “photos,” for example, they were told with a frowning-face icon that their friend would be personally upset with them.

“Obviously, it’s absurd behavior,” says Eben Moglen, a technology expert and professor at Columbia Law School. But should Tagged be penalized? Given that the site has already abandoned the recruitment scheme in question, it may be a tough case for the state to win, especially since Tagged’s practices overlap with those of other social-media sites. Says Moglen, “These things aren’t decided by building inspectors. They depend on what the judge ate for breakfast.”

Experts suggest that the case seems destined to be settled out of court. That’s still a win for the attorney general, who can reap the political benefits of filing the suit. But it does serve as a reminder that in this new realm, the onus is on consumers, not the established law, to help decide what is safe, sleazy, or just plain silly online.