Social-networking Web sites like and have helped link millions of friends. But now they have a new enemy: 20-year-old Jenny Thompson.

After Ms. Thompson created a MySpace page two years ago, she found herself sifting through dozens of requests daily from would-be acquaintances seeking to link to her page. By early this year, she’d amassed 4,000 such "friends," most of them strangers. Many flooded her page with remarks like "omg" — shorthand for "oh my god" — "you’re so beautiful." By June, Ms. Thompson, who resides in New London, Conn., was sick of the comments and posted a farewell ode before deleting her page:

"good bye myspace.

I’ve always hated you.

I just never had what it took

to leave"

Ms. Thompson belongs to a fringe of Internet users now renouncing MySpace and other social-networking sites — not in spite of their popularity, but because of it. That highlights a dilemma facing News Corp.’s MySpace and Facebook Inc.: While it takes a critical mass of users to make these sites work, having too many users alienates some, especially when they attract an ever-growing cacophony of advertising and in some cases, spam.

Both MySpace and Facebook lost visitors in September, according to Nielsen/NetRatings, a Web-tracking service. The number of unique U.S. visitors at MySpace fell 4% to 47.2 million from 49.2 million in August, and the number of visitors to Facebook fell 12% to 7.8 million from 8.9 million.

Charles Buchwalter, an analyst at Nielsen/NetRatings, says the slowdown is seasonal; last September, both sites also lost visitors as students went back to school, but then rebounded. Facebook spokeswoman Melanie Deitch says the Palo Alto, Calif., company’s own numbers show that the number of registered Facebook users rose 9% to 11 million in September, while the number of individual page views rose 40% to 16.5 billion.

Meanwhile, Judit Nagy, vice president of consumer insights at Fox Interactive Media, which oversees MySpace, also says the drop in visitors in September is seasonal. But she acknowledges that MySpace is "moving from a growth spurt into a phase of maturity."

Neither MySpace nor Facebook will disclose the number of people who have deleted their pages, but a MySpace spokeswoman offers that there has been "absolutely no increase in the rate of deletions."

There’s no question, however, that MySpace’s recent popularity has brought with it a proliferation of spam that has annoyed some users. Many advertisers take advantage of the "friend request" function and send out requests that are really just advertisements. And programs have cropped up that can automatically send mass friend requests to MySpace users — in short, a new generation of email spam. Sites with names like and sell the programs starting at $19.95.

The guerrilla marketing has driven away James Kalyn, a 30-year-old technical writer in Regina, Saskatchewan. He kept receiving friend requests from half-naked female strangers through his MySpace page. Clicking on a request usually led to a profile that turned out to be an ad for a pornography site. At first, Mr. Kalyn was excited that "these hot girls allegedly wanted to be my friend." But after looking at a few profiles, he realized: "If it’s a picture of someone fairly attractive, they’re probably not my friend in real life." Last spring, Mr. Kalyn killed his MySpace profile.

MySpace says it has incorporated technology to identify and block spammers.

Facebook has so far avoided a spam problem. But it alienated some longtime users when the site — which was once the exclusive domain of college students — announced last month that anyone can now belong. Nearly 3,000 Facebook users have joined a group called "Official Petition to Keep Facebook Limited to Students." A note on the group’s page reads, "Facebook just opened its doors to everyone on the internet. That means your mom, your boss, and every stalker in the world can now make an account."

Meanwhile, another key selling point for Facebook — that it lets people connect online with people they know offline through friend requests — has turned off some users, like 19-year-old Julie Miller. Ms. Miller, a sophomore at the University of Missouri in Columbia, Mo., dropped Facebook earlier this year after a few sketchy experiences left her feeling uneasy.

One day after dinner, for instance, a fellow student approached her outside her dining hall and asked for her number. She dodged the question, but by the time she walked back to her dorm, the student had already found her on Facebook and tried to add her as a friend. Ms. Miller says it made her feel uncomfortable that a near-stranger could track her down so easily and so quickly, thanks to Facebook. (The man knew her name and would have been able to search for her on the site.) The experience was "really creepy," Ms. Miller says.

Facebook’s Ms. Deitch points out that the site still has privacy controls that hide users’ personal information from the vast majority of strangers.

The loss in unique visitors comes on top of several months of slower growth for both sites. Traffic to MySpace inched up 3.1% in the most recent three-month period ended in September, compared with a 45% jump in the same three-month period a year ago, according to Nielsen/NetRatings. Facebook’s traffic fell 1.7% in that period, compared with 11% growth a year ago.

Some slowdown is to be expected for any large site that may have reached saturation in the U.S. A site like Yahoo Inc.’s, for instance, sees fairly steady traffic month-over-month but not much growth.

Advertisers and big Web players seem as enthusiastic as ever about social-networking sites. Google Inc. in August agreed to deliver at least $900 million in ad revenue over 3½ years to News Corp. for the right to broker advertising that appears on MySpace and some other sites. Microsoft Corp. also recently struck a deal to be the exclusive provider of advertising to Facebook, under terms that weren’t disclosed.